Agreed on timing being the first suspect.
When you say the guy who worked on it is "fairly confident" he has the pump timed correctly, what do you mean? There's nothing to be confident or not confident about... It either IS timed correctly, or it isn't.
There's a specification and a process, involving a measurement with a dial indicator. You want to find out whether the guy who did the work has the the right equipment and knowledge, and if he understands how it works, and what their actual timing measurement is if the answer is yes.