Hi Tony,
If both sticks are running OK on their own you have a memory refresh problem due to the mismatch.
Go into your BIOS startup screen and select a lower memory refresh rate. I don't know your Mboard, but you will either have options like "refresh = NORMAL / FAST / SLOWER" or CAS = 2/3. Go down to Slower or CAS = 3 . This should fix it.
If not get hold of a memory tester and run that. Lots of free ones on the web, seek and yea shall find.
Hope this helps
MG