For a long time my 94 Pontiac G Prix was misfiring when I got up to around
80 km/hr going up a hill. No one seemed to have a solution. The only way to stop the bucking was to ease off the peddle and give it some gas so it down-shifted. Whenever it reached 80 it would start again.
I decided it was time to change the plugs and discovered the cause. One of the plugs wasn't seated - it was loose! What was happening, it appears, was that when the pressure reached a certain level I must have been getting a bit of blow-by. At least that's my theory, because after I changed the plugs (which were in reasonably good shape) the problem disappeared.
Ya never know!!
Keith P.