2005 Pontiac Wave Engine stutter

Loosing power when going uphill but ONLY if im going 85km/h (within that gear range.)If I give it more gas to make it gear up then the problem disappears. Its only if im going around 85km/h or 55m/h. The car jerks a few times depending on how much gas I give it within that gear range. My mechanic suggests I replace the wires and plugs (i do not know when these were last changed) and I am going to do this anyway seeing as I am out of options. Any ideas?

The problem only happens at that speed; if im going slower or faster up the same hill with same load it does not seem to happen. Ive put "seafoam" into the gas to try and clear any clogs that may have been in the fuel filter or gas tank. (I thought it was the problem). Fuel filters are inside the gas tanks of these pontiacs.

Lastly, ive just had the timing belt assembly (belt/tentioner/harmonizer etc) changed along with a strut mount if any of that matters. The car was sitting for a few months before I bought it.

Thanks for your help in advanced,

~Devin