My car keeps on loosing power. 10 mph max speed. If I pull over and switch the engine off for a few minutes, the car will run fine again. Then the problem will return, at no specific time or mileage. Could run fine for an hour, or it could return after only 5 minutes. Its a turbo diesel engine. Seems to be an engine management problem, no faults flashing up on the computer however. Any good advice please.