Help! Intermittent fault

[Volkswagavin]

My J reg 8v GTI has developed a grumble.  Occassionally when accellerating (so far happened in 1st 2nd and 3rd) the car feels as though it is being starved of fuel or air and although the revs climb as normal, my speed doesn't.  It feels almost as though it has a carb that is icing up!  Strangely, when I apply the clutch and release it whilst in the same gear, it pulls as it should.

I've recently had the plugs, leads rotor and dizzy replaced, but this didn't make a difference.  The car is run on either Optimax or Tesco super unleaded, but the problem has occured with tanks of either.

My earth strap is looking a little worse for ware although it is still hanging on - could it be that? Why would it only happen sometimes!?  For instance, it was fine on the way to work this morning, fine yesterday but Sunday it gave me a fair amount of grief....

Any ideas?

[Bazzer]

I may be reading that wrong, but if your revs are rising and your speed isn't it's your clutch mate...

Or the cable / adjuster? 

[Mk2Kezza]

I would also say that your clutch is 'slipping'. If your engine is revving as normal and your not going anywhere (especially in lower gears) then the power isn't being transfered from the engine to the gear box. As Bazzer suggested have a look at the clutch cable first - it may be knackered, or sticking as when you dip the clutch a second time you may be loosening it and the clutch may then be biting. I'm no expert but it sounds like the clutch.
Best bet is to keep checking this forum, until a more experienced VW nut can give you a better explanation.
Good luck.

[rubjonny]

Replace the clutch cable with a new manual one before you do anything else
Try Halfords as last time I checked it was discontinued so they were going for a fiver each.  Ask for a manual cable for a 1.6 driver otherwise they'll try to sell you an auto adjust cable!

[rowlers]

I've had this also, but mine was only doing it in 4th and 5th! revs rising, speed not. Tried clutch cable and failed, ended up fitting new clutch @ £250, all is now fine and dandy.

[Volkswagavin]

I've just been told by a local VW nut that its a vacuum leak in the fuel injection system.  It causes a loud whining on accelleration and sporadic flat spots!  Apparently there is also a valve that gets gunked up and that doesn't help matters.

Phew!

[Judderi]

That would probably be the ISV idle stabilization valve.