BSOD differents bsod ntoskrnl.exe in many months

Hello,
some days ago a friend took me his pc because he had got many BSOD (I posted yesterday about that with no luck), many of this BSOD are happened after I have installed in that pc an ATI HD 3650 PCI.

First test group on this machine (Acer T650-99TZ):
prime95: rounding was 0.5, expected less than 0.4 (after 2 minutes)
memtest86: many errors whit 2 stick of ram, 0 errors when testing the sticks singularly
Furmark: test passed with no errors (30 minutes)
hdtune: no errors

Second test group on second identical machine (Acer T650-99TZ + hdd, ram and gpu from the first machine)
prime95: rounding was 0.5, expected less than 0.4 (after 2 minutes)
memtest86: many errors whit 2 stick of ram, 0 error when testing the sticks singularly

Third test group on second and first machine (same hw except for the gpu: here I took out the HD 3650 and used the integrated gpu)
prime95: no errors (in 8 hours)
memtest86: no errors (100% passed)


Ram: 2x1gb 1Rx8 PC2-5300U 555-12 hymp112u64cp8-y5
CPU: Pentium 4 524
Obviously no overclock was made on this pc.
Obviously gpu drivers are up-to-date.

Now i'm trying to donwngrade the drivers of the GPU but I think this won't work, since the problem appear to be an incompatibility between the HD 3650 and the other hw.

Can anyone help me to solve this problem? Thanks