Roger I'd very much like to help you but I'd need to do some examination of what exactly is causing the problem.
You're saying
Unless I stop the avast On-Access Protection...
and my question is how do you stop it? And what happens if you only stop part of it? The goal is to find the components that's causing the problem.
The on-access scanner, the VRDB generator, the auto-updater, the GUI synchro-timer -- all and any of them can be guilty here... You see, avast has a number of background components and it's quite natural for them to do some work, from time to time

... God knows what exactly is messing up the power management timer...
Thanks
Vlk