Engine Performance problem
2003 GMC Safari 6 cyl All Wheel Drive Automatic 105xxx miles
I purchased the vehicle in missouri, had a complete tuneup w/ gm replacement parts, wires spark plugs etc. I moved to colorado and when going up the passes its seams very weak the light for the check engine light came on. My friend hooked up a tester it said the second cylinder missed fired. He checked it out said the gap was off, corrected the gap and put back on. It still comes on when I try driving over passes and the pwer seems week, the light goes off after I decend and back to normal highway the power is enough when driving around the normal interstate, except for the passes any help thank you.