Strange System Restore behavior

System restore is acting really strange on my system to a degree I've never seen before. I 1st noticed it about a week ago. I had roughly 320GB free on my hard drives and all of the sudden over a weeks time it would continually drop. Day 2 it went to about 305GB, then 299 to 285 etc. until it got down to 261GB free. I wasn't downloading anything, making any changes I was merely moving data between 2 NAS's and some simple email, web browsing periodically etc. I could literally just refresh the my computer window and see gigs disappear right before my eyes. I tried several virus scans, win defender, avast, malwarebytes, ccleaner. nothing found. I cleaned all temp files, browser history, recycle bin etc, I may've gained a gig or 2 only to watch it disappear shortly after so I looked at my system restore. It was set to use all available space and all the restore points looked fine so I cleared them all and all my hd space returned. So I set it back to only use 10% and saved and closed. A day later I saw the hard drive space disappearing again and went back to restore. It was back to using 100% with no new restore points but it let me delete again and I turned off system restore. Since I turned it off my free space remains. Other than this my system runs fine, no lag, never slow, nothing odd in the windows task manager and no errors of any kind. Is this a common issue with system restore that any of you have seen before? It has saved me in the past but not for a while, should I just leave it off? Thanks for any info provided. I'm running Win 7 Ultimate x64.

Thank you for your reply. I completely removed Avast and as much remenance of it as I could with CCleaner and by searching out everything with *Avast* in it then rebooted. Before I could reinstall, the system restore issue resurfaced and within the 1st 10 minutes of a fresh reboot and no active AV, I had already been knocked down from 320GB to 309 free space. Checking system restore, it had reset itself back to using 100% of drive space and was reporting a mere 80MB of space being used, but there were no restore points saved. I then hit delete all points and turned it off again and went back from 309 to 320GB even tho restore said it was using 80MB. This is very strange. As of my writing this, I've reinstalled Avast and have done a full scan. Nothing found. Restore even turned itself back on and is still running amuck

MSE and Panda Cloud are very good free AV's, Panda a bit more aggressive.

This has only shown up recently that I've seen. Restore space should never be more than 5-10% unless you have the space to spare and reason to want more.

I have an Avast Pro license good thru 5/2015. I may bite the bullet until it runs out and try something else. What gets me is that restore was still acting crazy even when I had Avast uninstalled and wasn't running any active AV. Very strange. I did set restore back to 5% from 10% about half an hour ago and its been quiet since then, but I'm sure that's the calm before the storm.

I didn't know there was a removal tool, I didn't use that. Is it part of Avast itself or 3rd party?
I do have several cloud accounts, but I don't use any on my PC. I have 2x Synology NAS boxes that I use to handle them and I didn't notice a cloud account with Avast, nor did I sign up for one but I'll check thru the settings and see if there is something there.

Thank you, I used the tool and it seemed to remove Avast. Additionally, I had both my NAS's shut down and there are no cloud clients installed on my system yet the problem remains. Even prior to my reinstalling Avast again.