If it's a fresh Avast install (and I guess it would be) it's possible that is the cause.
While offline, run an application (movie, whatever) that prompts this jerking/freezing. Try disabling the AV for 10 min., and see if it still happens. If not, that's probably the reason.
The cure is to run a full virus scan. Once Avast has learned the contents of the drive, it will not scan everything running so remorselessly. (A simpletons explanation. Me, not you.)
In the Avast GUI, set it under "RealTimeShields>FileSystemShield>expert settings>advanced" to tick all three checkboxes, if they aren't checked already.
Check in all the shields that have them that the heuristics are set to normal. (File, Webshield, and mailshield, I think.)
If disabling the AV made no difference, I'm a little stumped.
I have visited PCPitstop in the past, which offers a battery of disk and processor health checks that I've found pretty useful for running diagnostics and muddling through problems, or just improving performance. Naturally, they want you to purchase some of their software (or partner software; reg cleaners, driver agents etc) but if you know how to interpret the results you might find it reveals the problem. You do need to create an account. I've never been spammed by so doing.
Welcome to PC PitStop! Only works in IE.