Engine Performance problem
1998 Chevy Lumina 6 cyl Front Wheel Drive Automatic 226000 miles
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(3.1 Liter engine) Idle constantly bounces from Hi idle to VERY low idle (so low it's about to stall) and back again, spending about 1 sec or less at either speed.

Engine Runs fine as long as it isnt at idle. Engine is also getting hard to start.

Replaced serpentine belt, Air Filter looks good, Fluids OK, except oil, which is black & slightly low. So I will change oil & filter it this week, & maybe add some stp because engine burns a little oil @ startup.

I've done work on 70's & 80's gm vehicles, so Im not afraid to do DIY repairs, & I have the GM service shop books for this Lumina, but not very experienced on the latest technology yet. Some tell me this crazy idle shows the computer is getting confused because of bad input from somewhere?