HELP! Computer only works with 2gb ram not 4gb

i bought this ram http://www.newegg.com/Product/Product.aspx?Item=N82E168... and put it into my old mobo http://www.newegg.com/product/product.aspx?item=N82E168.... for some reason it is only when i try to use all 4gb of ram does it freeze and not work. both sticks run fine on their own on win7 64bit (i am writing this message with 2gb right now) if there is some kind of compatibility issue that i have missed please tell me. if you know what could be the possibly problem that would be awesome to know. i have run memtest86 on both sticks individually and as a pair with no problems coming up. although i did try using the microsoft version of ram testing and it froze with the 4gb ram and when i rebooted said there was more then 4gb to test and thats why it froze. i have tried leaving the timings to auto and tried the 5-5-5-15 timings (cpu-z said it was 5-5-5-31 and tRC at 23). i have also tried using the other ram slots and still it does not work. HELP ME PLEASE!!! thanks for all your answers.