'90 V6 Firebird Idling problem

So straight to the point. My car is a bone stock automatic. When I put it in gear (reverse or forward, just as long as it isn't in park) and continue
to idle, my car begins to idle roughly. When I turn on the A/C a similar thing happens, but rougher. Also (although possibly unrelated), when I sit for a while (like at a Drive-Thru) or if I am going from 70 mph down to 35 (getting off at an exit from a major interstate, for instance), my SES light comes on. When I'm idling, I can predict when the light will come on because the rough idling I mentioned earlier eases up a little bit, and the engine's idle sound becomes slightly higher pitched and slightly smoother. Anyone have any guesses as to what kind of problems my car is having? Thanks. -Ben
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FSUguy wrote:

    You need to have the trouble codes checked. It could be a number of things causing this. Charles
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I was wondering; does the computer store the codes after the SES light goes off? Because it only stays on for about 15 seconds or so before it disappears, which makes me think that doing the self diagnostic would be a waste of time. So should I go ahead and try it anyway tomorrow I guess?
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It will store the codes until the battery is disconnected. I'm betting it has to do with the O2 sensor or coolant temp sensor.

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