A work colleague with the same car as me (2.0 TS 16V) had the same problem and it was the electrical connection at the back of the instrument pod. I think he disconnected it, cleaned and remade it and that did the trick. I'll pick his brains when I see him next.
Unbate your breath...spoke to my man Stu and he said he undid the 2 allen screws at the base of the instrument pod, lowered the steering wheel and pulled forward the pod. He remade the little multiconnectors at the back and then also there are little metal posts by which the individual instruments (rev, speedo etc) attach to a printed conductor track. He undid these, cleaned the points of contact a little and then put them back together. The rev counter receives its signal direct from the ECU. After putting everything back in place, he found to his relief that this had done the trick. I hope this does the trick for you because otherwise it's expensive by all accounts . Good luck!
same problem for me when i bought my 155 1997 2.0, mangoletsi couldn't find a cure so replaced under warranty - thank God - else would have been £400+
it meant a whole new instrument binnacle and zero miles on the clock therefore