iMac 2011 memory problem

Hi, another problem with my new iMac (am I cursed?)
Got entry model and get 4 gigs extra memory... system crashes from time to time, if working with powerpoint, crashes every few minutes... i tried apple HW tests and it found this error: 4MEM/64/40000000: 0x883bda 18 - its memory problem (as apple support told me). Then I tried this:
Inserted just original apple RAM (2x2 gigs) and Apple HW test ran perfectly, then inserted one 3rd party module (3x2 gigs), again no problem then I tried second 3rd party module (i switched one third party for the second one - 3x2 gigs) again no problem, but after testing all four modules inserted, same error.. what the hell is going on here?? any ideas? thx

btw: situation described is after changing first 3rd party modules I got (they were causing same problem - system crashes randomly)