GET A BOSCH CPS if you buy a generic one, it can waste your time! We've seen this before.

You MIGHT consider a new battery, too. Probably is the CPS but bad batteries or corroded terminals have causes some problems with the engine running right.

ALSO, re-soldering of the Relay Control board is not a bad idea, either.

IN order of likely-hood: CPS, Relay Board, Battery

Do you have a check engine light on? If so, reading the code may tell us all we need to know.