From stack trace, this could be faulty ram or video memory. But this is just my preliminary finding since only one dump file, test your ram and graphic card and please post more dump files 

Hi spikeee, I moved the RAM to other slots and also replaced a known good RAM to test. The problem of restarting still exists. By the way, I've reformat and reinstalled windows...still occasionally restarts.

Could be due to faulty memory, check your ram

Thanks for the dumps, 

Three of your minidumps shows sign of faulty memory with bugcheck 4E, C2, 8E, and 50. One shows avgntflt.sys with bugcheck 8E - a part of Avira AntiVir Personal, other shows problem with windows file system.
My suggestions:
1. Run chkdsk /f /r
2. Uninstall Avira AntiVir Personal
2. Uninstall and reinstall display driver with newest version.
3. Reset cmos, move ram to another slot. If still BSoD, get a known good and compatible ram to test. If that doesn't change anything, stress test that good ram with memtest86+ (refer here www.memtest86.org).  If you get error then I believe your motherboard is failing. If we want to make sure, please format and reinstall windows for problem isolation hardware/software.

agree that...

try download Ulitmate boot CD (UBCD).... google it... burn  it... on cd
all tools inside.. great tools. I still using it since 2003

Before do that try do some physical check.. some motherboard had capacitor malfunction.
Can visible when see some mud on top of it. Some case been solved before...