I have a 1997 gmc sierra with 190,000 miles. It has been running great till went on a road trip. About 6hrs away. Then it started running rough. So had new plugs put in new distributor cap and rotor. Also new fram air cleaner. Still running rough. Seems to run smoother when the ac is on. Is there a way to check computer diagnostics through the heater display?Any other ideas?HELP!