Another possible clue is yesterday with the buffer size at 516 samples, I played back a project with 7 audio tracks. It was faltering badly and the individual track VU meters were flashing on and off until they all stopped working. This happened in both the Tracks and Mixer windows. The tracks continued playing but faltering and clicking all over the place.

My ASIO driver offers sample sizes in increments from 49 up and I've now tried them all. It's now set to the highest, 1064. There is no discernable improvement in playback and all the other problems (including the above) remain and the beep that became a buzz at 512 is now at a lower frequency, a kind of of vibrating sound.

Whenever I do an action whilst playback is in progress I get this buzz and playback momentarily stops at the same time. This includes when I open the mixer or audio edit windows, drag a window to another position or try editing effects in the DirectX/VST window. Obviously all this makes editing and mixing pretty impossible.

I've disabled the internet connection and the anti virus software in case one of those was running something in the background that's interfering. I've also checked the Task Manager to make sure the only application running is PT and monitored CPU useage as it's playing back, which generally ranged between 2 and 10% with an occasional peak around 20%.