random crashing - but whats going on?!

[benghis_kahn]

problem started whereby computer justs shuts down, for no apparent reason, and rather randomly. usually happens when plying games, so got RMA no. to check: no errors (and have proven this elsewhere). then thought power, i.e not enough to power gfx (x1800xt), so got a Corsair VX 450w at weekend: no difference! finally thought RAM. one stick gave no errors/crashes all day; the next caused loads. tried different combos of DIMMs/slots, then ran memtest86+ v1.70 on each, as outlined below:

run DIMM 1 ('good one') in slot A1 and B1.
also run DIMM 2 ('bad one') in slot A1 and B1.
-> No errors at all whilst running single channel mode.

btw, my mobo (asus p5k SE) manual shows this:
single channel = B1 or A1 (apparently in that order?)
Dual channel = A1 + B1, or A1 + A2 + B1 + B2

put both DIMMs back in and errors appear again.

this obviously seems to be Dual Channel related then.

out of interest, my mobo does have the function of automatically reducing DDR800 CL4 ram to 667mhz as default, but it is perfectly OK to manually change back to timings to 4-4-4-12 (which i have done)

anyway, memtest results shown below:
i am getting 48 errors on test 5 and 1 on test 6. and the failing address is showing as 000fee0000, 4078.0MB, 00000000 (good), ffffffff (Bad) and Err-bit:
fee00004
fee00008
fee00010
fee00020
fee00040
fee00080
fee00100
fee00200
fee00400
fee00800 and on...

useful info: Asus P5K SE, Intel C2D E6750, Geil DDR800 2gb dual channel kit, X1800XT, Corsair VX 450w, WD 160gb SATAII.

Any ideas? as you can see, i have tried loads of things, but now getting confused

Thanks in advance
Ben