Sorry to hear about that unusual behaviour. I think it may be quite difficult to pin down the cause. The simplest solution might just be to have VC restart itself every 3 hours. I guess it depends on how predictable the timing is. If you are unsure how to do this I can send you XML for a command.
In order to try to find the problem, you could try to disable the plugins, one at a time and see if it makes any difference. Then at least we would know if one of the plugins is responsible. I run Sonos plugin 24 hours a day and I don't normally have any problems.
I have seen the CPU spike before and restarting VC fixed the problem, but I have not seen that activity for many months (I can't remember the last time but it was a long time ago) and I run VoxCommando on my HTPC all the time that the PC is awake, which is most of the day and the evening.
You can try to enable logging and send me a complete log, telling me at what time the CPU activity goes up, but I suspect that this will probably not show us anything useful.