shaking GTR

Hello, can someone help me please, i just recently acquired a r33 GTR. It was working fine until the water pump went, i had it replaced but now when ever i drive and accelerate hard, the car goes fine until about 4500 rpm, then it starts to shake bad. However when i accelerate slowly, it can go to 8000 rpm smoothly.. any ideas on what is happening (i'm short on money after replacing the water pump, so can't get a mechanic to look at it).. Any advise is appreciated by this GTR newbie

If the shake started after they did the work, take it back to them, so they can go over there work. It sounds like the harmonic balancer, but it would shake no matter wha rpm range you were at. That is very strange if you do it smoothly it wont do it?

It would have to be something they did. Did you look at there work, make sure everything was tight.