I've had an A4 Avant S line 170 for about 6 months, love the car but ...

 

For a few months now I have been experiencing an intermittent fault. Only seems to happen on long journeys. The glow plug warning light comes on after an hour or so of driving with a serious loss of power. Stopping the car, switching off the ignition sometimes cures the problem as it did yesterday while driving up north on the M6. Audi main dealer had the car in (again) last Tuesday and ran all the diagnostics costing me a hundred quid but couldn't find any fault. Anyone had the same problem?  

Does the light flash or is it a solid light?

It flashes

Hmmm seems strange that the Audi dealer was unable to find any error codes. A flashing glow plug light would usually leave an error code...

 

I did read somewhere else about a guy with a similar issue and it turned out to be a garage had filled his oil level significantly too high and it was causing oil to get into the manifold and putting the car into limp mode.

 

I also know that the actuator and boost pressure sensor can be common faults on your model that could cause similar symptoms.