hes saying when you overclock you stop when you cant get any higher without crashing. the Max memory speed you got without crashing was 1250 so use that.
if you tell us your power supply we may be able to.let you know if you are overloading it, and causing the crash. but I doubt it since this is the memory.
Maybe it`s my processor (AMD athlon dual core 64 2,9ghz) or my memory (2X kingston`s DDR3 2GB, it has i little low rate per second)