100% errors from memtest86 after series of power surges; more likely to be motherboard or RAM?

Solution 1:

First, check that all parts are seated correctly. One RAM stick slightly askew in a socket can mess up memtest.

Second, have you made a bootable USB device, using Ubuntu Live, Ultimate Boot CD or one of the many alternatives? Boot from USB and run the memory test from that, eliminating the HDD and/or OS as the issue. For example, on Ubuntu, run memtest86+.

Also, if you can borrow a similar PC to test the RAM, check the sticks there: It seems unlikely all would be completely bad. If they're OK, then some hardware on the motherboard, or the power supply, is the issue.