car sometimes jerks while coming to a stop

I have a 99 540i auto. Once in a while when i'm slowly coming to a stop, maybe at around 5mph or less, the car would jerk to an almost complete stop as if i suddenly applied more pressure to the brake pedal. But at the same time the engine rpm would drop and felt like it went low enough to stall the engine but it never did stall on me. All these when it happened it happened so quick for a split second but everything always recovers to normal. I couldnt tell if it's the engine sudden rpm drop that caused the jerk.
I just bought this car not too long ago. Here's a list of the problems on my car that i'll soon deal with, and dont know if they have anything to do what the above symtom:

- DSC triangle, amber ABS and Brake light constantly on: previous owner claimed that he disconnected it. I did a little research on the forum, and i verified that the speedo and cruise control all works. The brakes appears to work as normal.
- cracks on flex disc, the plate where the driveshaft bolts onto. Car makes squeaky noise when it starts rolling from a standstill and sometimes when coming to a stop. Maybe center support bearing is bad also?
- Signs of slight oil leak by the transmission shift shaft (it's an auto tranny) will have to check oil level. Maybe it's running low and cause the tranny to act strange.

The car was on a lift once at Dinan for something else and the tech handed me a list of things that need attention and no troubleshooting had been done on anything at the time. Some items are just the typical stuff for an E39 with high mileage but the ones i listed above are what i think that needs immediate attention. I'm gonna take my car to a friend that owns a shop and i'll get to work with him together and look at the car in details. Otherwise the car starts and drives fine, no check engine lights and the engine runs normally. However i'm limiting driving this car to a minimum. Its my second car.

Maybe the torque converter (auto clutch) isnt disengaging smoothly.

Originally Posted by Topaz540i

Maybe the torque converter (auto clutch) isnt disengaging smoothly.

what other symtoms can a bad torque converter produce? I've recently noticed this problem where if i just tried to maintain vehicle speed with partial throttle and the engine is running between 1500-2k rpm in 5th, there's a chattering noise coming from like under the car forward of the shifter/firewall area. Usually pressing the gas pedal down a little more, it will downshift to 4th and accelerate and the noise goes away. Very occasionally though, when i press in a little more gas in this situation, the rpm would oscillate quickly within maybe a 100rpm without much acceleration and the car shakes(feels like someone driving a manual tran and drops the clutch a little too quickly). When that happens i just give it even more gas, it will downshift to 3rd and accelerate and the problem goes away. I'm not sure if i'm able to explain here exactly what i experienced , but in short i'm concerned about the chattering noise at low rpm while maintaining vehicle speed or slight acceleration. Other than that the tranny upshifts/downshifts fine in normal or spirited driving.

On a side note, the flex disc and center support bearing on the driveshaft had been replaced after i originally started this thread a while back. The old disc wasnt in a really bad shape but the center support bearing did need replacement. Also checked tranny fluid level and it was fine. Seemed to be changed fairly recently just before i bought the car. I had put maybe 1200 miles on the car since then.

There's also a steering shake at around 45-55mph. Guys at Dinan told me i had a bad control arm bushing without me telling them about this problem. My friend and i werent able to dublicate or pinpoint this problem while we had the car on the lift and tried to shake the front wheels. I'm thinking about replacing them soon anyways since it may affect safety. I should research on this more or start another thread.

Originally Posted by gl4832

DSC triangle, amber ABS and Brake light constantly on

I typed /abs F3 in the bestlinks and came up with many links - one of which is this:
- How to diagnose & fix the ABS DSC (or ASC) BRAKE trifecta (1)

Originally Posted by gl4832

There's also a steering shake at around 45-55mph.

I typed /vibration F3 in the bestlinks and found this which may help you narrow the issue down:
- The main causes of vibration while highway driving (0) (1) (2) (3) (4) (5); while highway braking (1) (2) (3) & why it's not rotor "warp" (1); and severe shuddering while slow speed braking on bumps (1) & TireRack vehicle vibration diagnosis chart (1) (jpg) with match mounting hints (1) & due to worn drive shaft, flex disc, center bearing, or "giunti Boschi", namely the giubo (pronounced sort of like jew'eebo), and sometimes unknowingly misspelled as "guibo" (1) (2) (3) (4) & how to repair the rear driveshaft seal by the differential (1) or the inner constant velocity (CV) half-drive shaft (1).

Originally Posted by gl4832

cracks on flex disc...Maybe center support bearing is bad also?

See above links.

Originally Posted by gl4832

what other symtoms can a bad torque converter produce?

I typed /torque converter F3 in the bestlinks and found this, which may help:
- How to diagnose the P0741 DTC (1) & how to replace your torque converter (1)

Originally Posted by gl4832

there's a chattering noise coming from like under the car

I typed /rattle F3 in the bestlinks and found heatshield rattles, DISA rattles, catalytic converter rattles, etc.

Maybe one of those is your issue?