mine does this also. probably moves up to 100rpm on bad days. took it in and whilst porsche acknowledged not normal, they ran full diagnostics and held car overnight to check again and they concluded unusual tacho movement but totally normal operations of engine. Also interesting test is to set your car on cruise control on flat road at say 3000rpm and I notice my tacho needle still moves around a bit at constant MPH. I am not stressed about it though, I have pointed it out to dealer and if it ever causes problems in future I am sure they will look after me given problem identified early on.

Cheers