Hi Sneaky,

sorry its taken a while to reply, but touch wood, it seems to be running ok at the mo! I removed all the PWM calls from the interupt and put them in the main loop and it seems to be working as it should.

When I get the project finally finished, i'll probably have a play by making the USB interupts low priority like Darrel mentioned previously in the thread.
If I do and its still fine, i'll pop a reply to let everyone know etc.

Mark.
(been having probs hardware side dropping 24v dc to 12v without either having to use a huge heatsink etc, as everything ive tried so far keeps popping lol, but will be trying a step down regulator as soon as funds allow as it needs a few other parts to go with it and a redesign of the PCB, then etch etc etc.)