The latest Memtest86 -- which they call "4.0b (server)" on their site -- does a much better job of stressing the RAM, by using all available cores.
(Suggest avoiding the Memtest86 4.0a version, and the Memtest86+ 5.0 beta -- I've found both to be flaky. Memtest86+ 4.20 is stable, but does not use all of your cores and therefore is not as good of a stress test.)