2002 Blazer Extreme

I've got an '02 Extreme Blazer and the SES light is on. It's been on and off for about 4 months now. I was originally told that bad gas was probably the problem, so with some gas treatment it would be solved. After adding the treatment, the light would eventually go off in a day or so. However it would return in a few weeks. This time with new symptoms. Here they are...

1. When parked and idiling, it idles roughly and threatens to stall completely.
2. When first cranked up, it idles very high.
3. After cranking, when I put it in reverse, the engine stalls or threatens to stall.

The SES light will come on and off at various times sometimes on for days then off again, only to return another day. Can someone give a diagnosis for this or any suggestions?

Thanks!

Welcome to AF!

First and formost, you need to get the PCM scanned with a scanner to find what code, or codes are coming up. Thats the only way to get this fixed without guessing.

Went today to get the PCM scanned. Scanner showed no codes available to diagnose. SES light was on when they tried the first time. Then I was asked to turn the car off to make sure the connection was good, then to turn the car back on and the light had gone off. Any suggestions as to the cause of this?