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.