First of all, try to get two sticks running, using slot1 and slot3.
Leave slot2 blank for now. You may be able to run at full speed that
way.

To run three sticks, might require using single sided RAM (which of
course does nothing good for you in terms of total RAM on board - two
double sided sticks will give more total RAM). Using a double sided
in the slot furthest from the processor, and two single sided nearest
the processor, is another potential config, again doing nothing for
you in terms of max memory.

This is absolutely maddenning. I ran the test and it passed 7 times
in a row. I reboot the machine and run the same test, and memtest has
errors. WTF?!!!!

-GV