1995 low rpms

[thishardman]

hey guys
so my 1995 318ti has been making some noises lately, the belts have been squeeling and im thinking the belts are just old and need replacing. the issue is the other day i turned the car on and i noticed the revs were really low when it was sitting in park, then it eventually shut off. It started right back up when i turned the ignition though. Then finally last night i was sitting in a drive through and noticed when i was just sitting there, the revs went real low and the check engine light came on. Then, when i pulled forward and brought the revs above about 1500, the check engine light turned off and everything was fine. The car itself is driving fine but i don't know what to do about the low revs and the squeeling belts. are they related?

thanks

[1996 328ti]

Could be a bunch of things.
Vacuum leaks
Idle Control Valve
Throttle position sensor