intermitant clutch slip

[lambert]

late model 200tdi manual in good mechanical order regularly maintained.

every now and again my engine revs rise witout the related increase in road speed. usually i can make this hapen on a motorway when 5 or 7 up and i want to pass  slower car, or when in 4th on a back road and i open the taps to keep momentum for a climb but in iether circumstance slippage is not guaranteed. equally with the big trailer on and loaded i can crawl up some very steep gradients even in low box and with the accelerator  wide open it does not slip.

any ideas welcome, as to my mind if a clutch is going to slip it will do so all the time not just when it feels like it.
many thanks in advance.

[crazymac]

What fuel are you using??

Reason I ask, is if i'm using a high percentage of veg oil and the weather chills, then I get what can best be described as clutch slip, bit I reckon its unburnt fuel or waxed fuel?

[lambert]

shells normal dino diesel.

[mark-d]

mine used to slip now and then for ages but i couldnt always make it do it, in the end it was doing it all the time and i had to change the clutch, make sure you change the clutch fork at the same time as the clutch