[comp_jas]

Is that different than the code scanner that Advance Auto will loan you?

I borrow their scanner and read the codes. The only code set it Misfire Cylinder #5.

My car is a '97 also with a randomly set CEL.

The car definitely runs rough for a brief period of time the code is valid. But what is causing the code? Something changes in the operating conditions as the coolant gauge moves just above the C mark.

I thought plugged EGR ports might have merit. A slow responding auxiliary air valve or might also lean things out.