rev counter not stable

I am driving a '94 Jetta 3 CLi exec (8v). My rev counter seems to sway between 1000 and 2000 rpm while idling. I have been to numerous mechanics who cannot seem to figure out the problem.

The following was tested quite intensely:

- idle stabilization valve

- computer box

- check for any air leaks in the engine

- check for any electrical wires which are bare

I even went to the extent of taking my car to an AA aproved workshop, and that also did not solve the problem.

Please note that the above occurs intermittently, and also previously the revs went up and down from 1000 to 5000rpm but recently it has gone down to 2000rpm.

If anyone has experienced a similar problem, let me know.

Thanks Bishen