hey bud, My wife had a similar problem on hers. We started noticing on a Heavy load start, The machine would freeze. SOMETIMES it would recover, and others it would just hardlock. Even after reinstalling, removing all the hardware but the video card, it still did it. Come to find out, there was a stress fracture on the CPU. after taking it out, I noticed it. So I changed CPU's and it was fine.
As for you changing AV's and It not doing it. Try this... There is a program called (heat up) which puts stress on the CPU. one program that has it is MBM5 (Mother Board Monitor 5). Problem is the last thing I knew they stopped working on MBM5, but I recall you can download it from from here,
http://www.majorgeeks.com/download.php?det=311. Thats the last known version they did. The site that used to make it, Is no longer up. The only thing you need to know prior to installing MBM5 is your motherboard model, OR computer model. (Acer). when setting it up, and you get to choose your settings, Before to click update, then move to next. Choose your setup you have.
Once you have it installed, goto c:\program files\mbm5 There (should) be a folder called heat-up unless they changed it. I haven;t used the program in a while because I haven;t needed to. Upon getting to Heat-up , Double click on it. Let it run for about 5-10 Min's. (your machine WILL bog down) It will be under 100% CPU usuage. Typically, (IF) your CPU is damaged from stress it will hardlock!!! if the CPU is fine, typically it won't. This is how I found her (my wife's) problem with her CPU. It that fails, Let me know. and I have a few other trick up my sleeve.
also, MBM5 is the only software that has heat-up. Or that I've come across. maybe someone on the boards has it, and can put just that file on a FTP for you to download. If i had MBM5 installed at the moment I would for you. If you like I can grab it, and get the file for you, and place it on my ftp.