A) CPU Intel E7200 2.53 Ghz stock cooling
RAM 2x1 GB DDR2 800Mhz A-data
MB Gigabyte EP43-DS3L
B) CPU Intel E7200 2.53 Ghz stock cooling
RAM 2x1 GB DDR2 800Mhz PQI
MB Gigabyte EP35-DS3L
C) CPU Intel E7200 2.53 Ghz stock cooling
RAM 2x1 GB DDR2 1066Mhz Corsair
MB Asus P5QL
They all run smoothly with a single memory stick (either one of them). A and B can run with two sticks but aren't stable, C doesn't boot even.
I can't say for sure for the others but when it comes to A it definitely isn't a power supply issue, I guess that isn't the cause for the others too.
Only thing in common is the CPU, how can I check if that is the problem?
If additional information is needed I will happily provide
Using the part, model or serial numbers, find the exact same ram online, now check the voltages and timings for them, make a note of this for each stick.
Now for each system, go in to the bios and manually set the timings and voltages as advertised.
Not sure about the memory mapping thing, but for the two pc's that boot, download "memtest", create a bootable CD and run this on them.
If your still having problems, run memtest again using only 1 stick in slot 1, test again with that same stick in slot 2, continue till all slots have been tested. You will have to do this for each stick of ram. Preferably do this testing on a working machine, so that you know other factors may not add possibilities for errors.