The problem is almost certainly the throttle sensor - this is a common problem on (older) Ford Foci (particularly if you stamp on the accelerator lots!). Once the sensor starts to wear, the engine doesn't get the correct signal, and starts to run unevenly, sometimes idling too fast. Eventually the engine management system gives up, puts the engine into safe mode and flags up the error codes, causing the vehicle to lose power and the glow plug light to flash. Stopping and restarting the engine clears the problem, until it happens again.
The solution is to change the whole throttle (foot pedal + sensor), as the sensor is not sold seperately, but it only costs about £80, and is easy to fit.