no expert 'm afraid - so I've checked my vw chilton manual but no troubleshooting re Rev counter either.
If its operated similar to the speedo i.e., by a cable, then this might be faulty - speedo cables on VDO instrument panels on 92-93 Golfs (not sure about GTI's) were prone to cable wear and was noticable by fluctuating needle (sometimes did'nt work at all).
Anyway, I have'ny found anything specific on REV counter problems, but could be just cable/electrical - i suppose your vw garage must have checked wiring for damage or loose connection etc ..