This symptom is most likely the sensors on the accelerator pedal. If the three potentiometer sensors disagree, the computer drops the engine speed to idle, with glow plug light flashing, until switched off and on, then it may or may not be ok for a while.
Cost of new pedal, just over £50 a year or two ago. Time to change, maybe 5 minutes. (Just 3 small nuts and a plug.)
tiger99
October 2008