Memory error · DIGIT-ARC

I have an athlon 64 3200+,asus a8n-e mobo,with 512x2 hynix ddr 400 ram in dual channel,and a geforce 6600 128 mb running win xp pro sp2.My problem is, from sometime a memory error keeps occuring in my system with a system beep of One Long Beep, Two Short Beeps.I tightened the dimms at first and all went well but the error keeps occuring at random.Sometimes the computer does not start at all and sometimes it turns off suddenly and beep occurs.I also reinstalled the ram butt the problem still occurs at random intervals, I EXPERIMENTED A BIT AND FOUND THAT ON 512 MB RAM IT WORKED FINE THROUOUT. :?:

Don't know?Wired problem?Wait i will reply after i get it!!!!!

try this ,it might help
microsoft memory diagnostic "MTINST.exe" this program has to be install on floppy disk, computer will boot from it ,and start memory test.
put one memory module at at time,run the test.

after each memory module test, put both module together and run the test.