Having a few problems with my new car

[atomicradish]

I posted awhile back about getting a new car. It's a '94 Mitsubishi Galant GS. To get it in running order, we had to get 2 new timing belts put on it, along with having 5 valves replaced and a complete valve job. It drove really well for say 150 miles after we had this work done on it, but now another problem has arose. The car did sit for 6 years, so we did expect to run into a few problems...

When you accelerate hard, around 4,000 - 5,000 RPMs, the car holds back. Even if you mash the accelerator to the floor, it wont go any faster. The car lurches as though it is receiving gas intermittently, and it wont go above said RPM. It does not matter what gear the car is in, it always does this. At first I thought it might just be bad gas since what was in it was 6 years old. However after putting a lot of octane boost, and $10 worth of premium in it, it still continues, just as bad as before.

I doubt it is relevant, but this started to happen a little bit after changing the oil in it to a synthetic blend (valvoline 10W-40 max life).

If anyone can help... try to remember that I'm mechanically illiterate.

thanks