[jsinton]

They could rule out the timing belt and balance belt by doing a timing belt inspection. 50K is pretty close to needing a new one, but I'd want to be sure it would work before I let them do that. Could be a crank sensor problem, but I'd expect a check engine light if that was so. Maybe the crank blade has a crack, but that would be unlikely... but it would explain the problem.

Sounds like these guys are parts changers, I supposed you have paid for all the previous work, which did not fix the problem. Didn't they guarantee the work? Sounds like you're a goldmine for them and they want some more.