Memmory mixing probs

OK Just go a new stick of 2100ddr crucial ram. It does show in post in slot 2 but does in slot 3 however it comes up as PC1600. I've changed this in the bios to 2100 but its highly unstable in games and yields a lower 3D2001 Mark score.

Any way around this or am I a mug for not getting the same brand/type RAM

Aww sarf london geezah big ted, you are a mug for not getting same type!
What mobo and other memory module are you running?
Can you change memory timings manually to their slowest - ie Cas2.5, 7, 3 3 on both banks, speed set to PC2100...? Do you have any option such as "enhance DRAM performance" switched on in BIOS?