If you're not getting errors running Memtest, then I'd start looking at either the OS/Apps or other hardware. To properly test with Memtest you need at least 4 passes and with 16GB that takes quite a while. I would recommend that you run MSCONFIG and select Diagnostic and 'test' the system to reproduce the 'stability' problems.
On OEM MOBO's the BIOS is very limited and I doubt there's any settings to adjust your RAM.
Look at the Event Manager [+] Critical errors or the (.dmp) file(s).