I bought a new dCi 1.5 model, having SO MANY electrical problems ranging from invalid output voltage on the throttle potentiometer to invalid engine management and ABS warnings.
The problem you are experiencing is the ECU (Engine Management) going into "limp mode". A deliberate mode that will slow the car down to a safe speed that will get you home without a problem becoming fatal.
When this happens, a fault code or fault codes will be recorded in the ECU's NVRAM.
Get a decent car electrician to hook the ECU up to a diagnostic reader via the ODB Port (under the ash tray).
It should give you some insight as to what problems you're getting.
Or, like me, it'll reveal some complete random crap that bares no relevance as to why your car is broken.
ChrisR
December 2008