Memtest can miss minor faults so try running the PC on one memory stick at a time. Use memory intensive programs like low level games or watching/transcoding video for at least three to four hours with each stick, you may find that one of them will cause a crash.
The error codes can also relate to a faulty driver
, did you install anything new just before the crashes started, new software, hardware, driver updates, etc?
You should also switch on Windows Update from the Control Panel, you should be on SP1 by now.