I had avery similar problem with mine about 3-4 months ago, it was awful, it was almost as if it was running on 3 cylinders, juddering, spluttering and a severe lack of power !! I would stop, switch off and re-start and it would be fine again for 10 miles or so . . .
Anyway to cut a long story short it was the wiring loom that feeds teh injectors !!
Althogh the injectors are supplied with fuel at very high pressure from a mechanical pump, the actual burst of fuel and the timing of the same is controlled electronically !
Aparently oil seeps from the injector 'valves' and works its way up the loom until it reaches the connector block where it causes a bad connection ?
New loom (£34) 15 mins to fit and the problem is completly cured !!!
May not be what your problem is, but worth checking out !