So from what you have said it sounds like you might have some faulty memory there. Don't be fooled by the fact that you have replaced it as a percentage of RAM is faulty when purchased. The following tests do take some time but are worth doing properly.
Please follow these instructions
exactly.
I would suggest that you download and run the latest version of Memtest86+ to check your RAM first.
Memtest86+
http://www.memtest.org/Insert
one stick of memory in
slot 1 and run Memtest on it for at least
10 complete loops/cycles and if there are no errors then swap it over with the next one and continue untill you have checked all modules.
If you have any errors the module is faulty.
If you have any faulty modules you will have to return the whole kit as they are matched.
Post back when you have done that with the results.