Thanks again - I've tried selecting the various options I have in MIDI output drivers, including Microsoft GS Wavetable SW Synth, and none make any difference.
I tried resetting the Preload Track Buffer to 50 but it won't accept it - it says the range must be between 200 and 2000 so I set it to 200 but again no difference. Should I be looking for an ASIO buffers setting somewhere other than in PTs ASIO audio drivers window?