1998 GMC Sierra

I have a 1998 GMC Sierra automatic with 137,000 miles. The transmission was rebuilt almost a year ago, spark plugs replaced, wires replaced, water pump, fuel pump both replaced and radiator replaced. Fun year. When I go up a steady incline or hill, at about 70 mph, 4th gear and 2000 rpms, it begins to shudder. If I don't back off, it will shake. Consistently, the check engine light comes on and returns a " random cylinder misfire" code. If I try OD when it shudders, it jumps up to 3500 rpms then falls to 1500 quickly. My mechanic says the transmission is fine and can't find the problem. Also, recently, same scenario, only I had cruise control on. When it was on a hill and began to shake, I downshifted to 3rd and it was as if I turned the car off--it began to completely decelerate. I put it in park and everything went back to normal. Gas mileage is great. And I really just have this one issue. It's as if it need to be in a lower gear to get over the hill, but refuses to change. Help!
November 29, 2007.

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