This is a general MIDIclock problem. Most MIDIclock outputs (including PF) have quite a lot of jitter on them. Computers are usually worse. This is very hard for most MIDIclock receivers (including PF) to handle – they have varying degrees of success. I don't know of a solution to this – in your case you probably need to speak to the good people at MIDI Solutions (they can contact me for details of our MIDIclock out mechanism if they need – but it's pretty standard).
Our H8000 has dedicated hardware for MIDIclock generation, so its output is almost jitter free.