Roel, thanks for the pointers! This gives me huge re-assurance.
I’m no developer either. I’d say I’m competent and not clueless with this stuff but by no means an expert. Having tried all manner of advice around this issue - a lot of it from this forum - I seemed to have hit the wall when I wrote the post.
Your words not only tell me I’m not alone (ie it’s probably not something I can cure) but also indicate that there is an awareness and “hope” for the future. So thanks again.
Since writing the original post I’ve done a bit more investigation using MS Windows 8.1 Performance Tools. I think I’ve tracked the issue down to Deferred Procedure Calls. I can correlate a spike in the DPC graph for the IEE 1394 i/f with the peaking of the real-time VST performance meter and the audio drop out. That is not to rule out that the real culprit is the graphics driver taking resources away from the things we (audio application users) need rather more than 3D graphics.
I’m stuck between a rock and hard place with the graphics driver as the first one that is Win 8.1 compatible is version I have. There is nothing for me to roll back to.
If the problem is the IEEE 1394 i/f after all (and it used to be in the Win 7 environment), then I can’t use the legacy version (which was the workaround in Win 7) because MS haven’t signed their own legacy drivers for Win 8.1 so you can’t load them.
I’ve now come to the conclusion that the only route open to me (for now) is to revert the new machine to Win 7 (probably using dual boot). That way I can use older graphics drivers and the legacy IEEE 1394 drivers. My old machine ran Win 7 and, although less powerful than the new beast, never exhibited these problems with C7.5.
Once again, thanks Roel for your post.
James