Random Errors And Crashes Whilst Surfing And Gaming

This is a new pc build using various parts we have accumulated. When using Internet Explorer I get app crashes. I have been playing DC Universe Online and get random app crash, display drivers stop responding, apphangB1, I also lose monitor signal. Here is a list of fixes we have tried already:

-We have disabled internet explorer add ons and even tried disabling IE completely, to no avail
-We have swapped out the 5770 for an 8800 GTX that we know works, same problem
-We have the latest BIOS/drivers
-We have run SpeedFan, CPU temp 41C
-We have run Memtest for both sticks of RAM at the same time in slots 1 and 3 and there was an error, so we tried to isolate the bad RAM by testing invidivual sticks in slot 3 - no errors on either stick? therefore, we put the sticks in slots 3 and 4, assuming there may be a problem with slot 1 - worked ok for a couple of hours, then game crashes and black screen with sound requiring reboot. Monitor is fine, as it works on my BF's pc with no problems.
-We have tried a couple of installs of Windows 7
-We have run chkdsk - no errors.The BIOS is running on default, nothing overclocked - we have used the correct timings for the RAM and autotimed them, same problem

We are currently not using an antivirus because we did not want to add more problems, but we will get AVAST.My BF has given up now, as there is nothing else he can think of to try. I feel we are so close to resolving the issue, as it did run fine for a while, but then a bunch of errors all at once. If it helps, here is a copy of the error log.