bluestar

Hi All I have a strange issue that no mechanic can solve in my local area. The car is a 2008 A6 S Line 2.0 TDI. The car will be driving fine and then out of no where would judder more so going up hill and then stop dead, at this point it wont start at all just turns over. Found out that it blows a fuse in the engine bay (under the silver CPU right hand side of engine bay) the fuse is the middle of the 3 tried to find a diagram of what the fuses are but no joy and the garage had no idea. Change the fuse and the car starts. Had a good look to see if water in there but appears to be dry. Changed the fuel filter , popped redex in the tank etc and no joy just drives fine in high gears but in low gears seems to shake. No isea where to look as not a mechanic and can find nothing similar on the net or what the fuse is to trace back Any ideas ?