RAM Problem, Maybe!

I recently added a new set of 256 SDRAM made by Time to the existing RAM to an old comp. It worked perfectly fine for a few days but sometimes when windows starts it says it has recovered from a severe problem or Win Generic Host 32 crashes and then it stops the pc going online. Sometimes it just resets itself without any warning. Would this be caused by a RAM problem such as conflicts between different manufacturers RAMs or because Times RAM is S**t!!?

Thanks