Sounds like a ‘Funke’ issue to me, but I will be interested to hear what transponder (make and model) you are using, what the GPS source is and how they are connected.
I have considerable experience of running both FlarmMouse and PFC with both Classic and Rosetta with no issues, though as per Lee’s post, this should have no significance in relation to FR24 (unless of course you are using PAW or your PFC as the GPS source for your Mode-S/ES).
Regards
Peter