Wierd memory OC occurence on 5900se flashed to 5950ultra

Hey, last night I found a fedex package by my door. It turned out to be a 5900se flashed to a 5950 ultra. Needless to say, this made my day. It also had a zalman heatpipe with a fan and some memory sinks. I took off the heatpipe. I replaced the gpu interface with AS5 and the gaps between the heatpipes big metal sink parts with white silicon thermal paste. What was there before was pitifully inadequate. Anyway, I stuck it in my rig and tried overclocking it with the stock nvidia drivers. I can get 515mhz on the core which seems good. Anything higher and my video will lock up after a while. The memory would not go past 940mhz in windows without producing artifacts on the desktop. What happened next was really wierd. I can have the memory up to 1020mhz in america's army and have no artifacts. The only reason I can't go higher is that the drivers force you to test the overclock frequencies before it lets you set them. Since it artifacts on the desktop it won't let me go higher than 1020mhz before saying it is unstable.

I am just wondering what people think of this. Does this happen to anyone else? Is this a good overclock? I have absolutely zero artifacts in america's army. It is at 1600 X 1200 with every option all the way up including AA and AF.