Sounds like your ram is bad. I would go to
Memtest86+ - Advanced Memory Diagnostic Tool and download the ISO file and burn that image to a CD and then boot your computer with that disk and it will run a memory test of your memory outside if your Operating system. If you get any errors, its pretty much safe to say that you have a bad stick or faulty motherboard.. More likely the ram though..
I would start out by testing the sticks one by one, and if they pass, start adding one more at a time until your with your full load of ram..
But, its more then likely your RAM. I just had this same issue and got mine replaced and I've been good to go. The system may run fine because your not actually having to use the defective section of the memory.. i.e one of the various SDRAM chips on the memory, until you actually load a project that finally needs to use it..
Anyway, that would be the first place i would look.