Pump problem / pump repair help

[CJG]

I've got a sequence power series 7400 that I'm having trouble with. It doesn't seem to be running properly. The pitch of the motor seems to be going in and out a little bit. I know how the pump should run becuase I have another thats the exact same model that runs perfectly. Is there anyone here that knows how to break down a pump and look at it to see whats going on. I know the basics as far as getting into the front panel where the impellor is, but beyound that, if its not a dolphin pump, I don't know.

Here is a list of problems I know is not causing this.
- Its not electrical
- Its not due to sucking half water half air. The water running through the pump is at a constant.
-Its not due to the size of plumbing I used.