yet another weird start-up on 2003 Z4

I have had problems with the starting system in the past. I ran into a problem with the starter a few months back. The engine would not turn, just whine. I had the battery load tested before i tackled a bigger job. The battery is still in great shape. I replaced the starter and the car began to turn over just fine. I was having problems with cold starts, and as recommended, ran a bottle of techron through a 12 gallon tank of gas..I now only fill up with Chevron. With that being said, The battery, starter, and gunked up fuel injectors probably aren't causing this.

About 3 weeks ago, i went to start my car and the rpm's were jumping like crazy from .75rpm's to 1.25rpm's...It was very odd. After about 25 seconds it leveled out. about 1 1/2 weeks ago, I started the car and got an extremely rough idle. The exhaust was puttering. Engine was vibrating very strangly. Gave the car a light shaking feeling. I turned the car off and restarted it. Worked fine. Just 2 nights ago(on a spontaneous road trip to Galveston Beach), I stopped for gas and filled up the tank. I tried to turn over the car and it seemed as if it did nothing. I had the radio running for less than 5 minutes and thought i might have killed the battery. I stepped in with some cables and asked the teller if he had a car i could jump mine off of and turned back around and noticed a Ventilating Sound. At first, i thought it was coming from the gas pump but sure enough, it was coming from under my hood. The engine was vibrating and it sounded like the fan was running. After about 2-3 minutes of it doing this(regarless of ignition switch position) it stopped. I got back in and the car started fine.

Does anyone know what could cause this to happen?

Picture from the trip

im experiencing similar symptoms -- did u resolve this?

There is a problem with the motronic system. Happened to my e36. Also could be the sensors. Cam and crank sensors going bad

how much did you end up spending?

Fan running is normal. Erratic idle right after start up that resolves with a restart is often a cam sensor. They frequently go bad, cost around two hundred for the part, and it's an easy DIY. FYI, never run the radio, lights, etc on a newer BMW with the engine off. If you suck the battery low enough your whole car will run wacky til it's charged up and you will trigger a CEL. On my third bimmer - just my experience.

understood -and i did not run any electronics with the car not running...
the car was sitting for the entire winter with just a few starts here and there by the previous owners though ...