Power off the PC and remove the mains connection, always touch the metal of the case with your hands to avoid any static electricity from causing component damage. (On laptops ALWAYS remove the battery before removing or replacing memory modules). Remove all your memory modules. Clean the contacts with a soft rubber (up and down not from end to end) then insert just one module and reboot to test for any change in performance. Repeat this for every module one at a time and run for at least 24 hours with memory intensive programs, playing games or a video. Obviously if you only have one module this cannot be done, but you should still remove and clean the module and reseat it and test.
If you only have one stick download this to test it.
http://www.memtest.org/
You need to scroll down the page until you see the three blue screenshots. Just below is the list of downloads for Memtest86+ V4.10. Click on
Download - Pre-Compiled Bootable ISO (.zip)and save to your desktop, When the download is complete right click the file and select
Extract Here and burn the image to a CD. In windows 7 right click the extracted file, select
Open With, then select
Windows Disc Image Burning Tool then follow the prompts. For all other versions of windows (if you do not have an ISO burner) download this free software.
http://www.isoimageburner.com/
Boot the PC into the Bios setup and set the CD/DVD drive to 1st in the boot sequence. Insert the disk in the drive then reboot and the disc will load into dos. Leave the test to run through at least 8 cycles.
The memtest will not be 100% accurate but should easily detect any major faults.
