Bucking during exceleration

Buick Century 98. About 120,000+ miles Car runs fine, smooth etc. Check engine light always on, but doesn't seem to be a problem. For years.

Now, car almost acts as if it's a transmission problem as it bucks going up hills and upon exceleration, not bad bucking, but almost as if it's having trouble shifting to next gear.

Drive it slow or moderate. No problem. Only when putting some sort of load on the engine.

changed all 6 spark plugs, changed trans filter and fluid. Changed oil filter and fluid, was running hot, made sure radiator is up to par also.

Had a cracked head 5 years ago, had it re machined and didn't seem to have a problem until about a week ago. Still hesitating/bucking.

The problem started out as "running hot" thought it was the radiator. Could be a separate problem or they go together, not sure.

Help. I love my car. Otherwise it's in fantatic condition