About a month ago my car broke down, and I had to take it to a Chevy dealer since I'm out of state and don't know any mechanics up here.. They replaced the opti-spark, and it ran fine..

About a week later it started almost stalling or stalling while I was driving, and would start back up.. Took it back to the dealership, they said it was not the opti, that it was the O2 sensors and they couldn't fix it cause it was aftermarket.. They cleared the codes and it ran fine.

Fast forward a couple weeks and it's been almost stalling, usually at red lights and occasionally while driving at low RPMs in gear (ie 70 mph or so in 6th, coasting towards a light in 2nd or 3rd)... If it did stall out, it would always start again, and it seemed to do it less often if the A/C was turned off.. There is never a SES light though, which makes me think it has nothing to do with the 02 sensors as they would throw a code, wouldn't they?.

Today it stalled out and won't start again. Any suggestions? What could cause this problem and NOT throw a code...