DDR2 mixed up

is it possible that what cause my pc to crash sometimes is that i install 2 different DDR2 but same capacity 1gb each 667mhz both…one is nylex brand and the other is pcx given to me by a freind..it is detected by the computer itself but for a moment it will work then suddenly there will be a error and pc hang….then i will removed it the pcx brand pc is doing well..hows that?