I have the same problem on my 97 2.lT 16v drives me mad,I have to bang the dash to get it working again.Happens at random and I took out the instrument pod and cleaned the contacts all to no avail.sorry I cant be of much help but do many others suffer the same?
This is beginning to sound like a problem with the needle, two people have to "bash" the clock to get a reading. Perhaps a drop of WD40 in the right place might solve it....if you can get to it. Remove the pod and have a look, its only two screws!
Rather than start a new thread about rev counters I decided to do a search. Low and behold I find this one.
MY CAR IS A 1997 155 aswell !!!!!!!!! and I only picked it up today. Already the rev counter has stopped. Everything else is A-1 but it's nice to know this is a common problem I'll remember to belt the dash when i get into it next time
Definitely too much of a coincidence !!!!
EDIT: Without doing anything, the rev counter is working again. Obviously something dodgy in there