loosing power

hi, recently my 300tdi has been suddenly loosing power the gains it back, also the rev counter is playing up, cutting off then cutting in again, same with the lights going dim then coming back again. could this be an alternator problem?
cheers

it sounds electrical, would start checking earth straps and ignition circuits for corrosion / loose fastenings

I may be jumping the gun but it looks like a faulty ignition barrel... If I'm wrong someone will correct me

ok thanks, just seems a liitle weird it will just drop about 200rpm on idle and its sluggish when playing up, but then will come back normal, when i was driving in the dark the lights went bright and the engine sluggish then the lights went dull and the engine went normal. does it without the lights on too.

is it edc, or mechanical controlled pump ,if mechanical the only effect electricity would have is on pump solenoid ,rev counter runs off alternator so those are connected as would lights ,i would check alternator output ,