Possible Vacuum Leak?

If my '89 325i is going down a hill, and I'm not pushing the throttle, eventually the check engine light will pop on. It also happens when it's idling after it has been running for more than five minutes. The fault code is 1222, which is lambda control 1. This however, doesn't tell me too much, just that it's running either too rich or too lean for longer than 10 seconds. What is the best way to trouble shoot and find the problem?

To my understanding this could be a defective engine temp sensor, bad injector valve, incorrect fuel pressure, secondary air leak, defective air flow meter, bad compression, or bad intake valves. I would hope something as simple as replacing the O2 sensor would do the trick, but I'm thinking it's a vacuum leak somewhere and I don't know how to find it.

Another detail I forgot to mention, as soon as the check engine light comes on, any increase in throttle will turn the light off immediately. If I'm at a traffic light and the light pops on, a small rev of the engine turns the light off.

well the 2 tests that you could do to help diag the problem would be a fuel pressure test and a vacuum test. thatll knock out a few possible problems. the other thing, the car could just need a tune up. new plugs, cap and rotor.

It has new plugs and filters. All fluids are good to go. Cap and rotor hasn't been checked yet though. How does one go about performing a vacuum test and a fuel pressure test? If it's a complex answer I'll just Google it.

vacuum test just get the tool and plug a t fitting into a vacuum line and start engine.

fuel pressure is remove fuel line post filter (or entering fuel rail) put t fitting in and attach tool and start engine.

they are pretty easy tests. hardest part is getting the tools.