Have you tested all the memory slots one at a time with only one memory chip at a time? (one chip in slot 1, reboot then same chip in slot 2 then 3 then 4 etc. Then try a diff chip same way.)That way you can see if it is a bad chip or a bad memory slot or both. I'm guessing you might have a bad slot on the motherboard. 

 Memory slots are 1 & 2 (matched pair of ram for the 7100) and 3 & 4.  Put the same ram chips in 1 & 2 and the other set in 3 & 4. You don't have to match memory but matched sets run better since different ram will run all the memory at the slower speed. That is why you should have 2 sets of matching ram--all 4 the same size, speed and brand. Here's the service manual: http://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_studio_xps_desktop/studio-xps-7100_service%20manual_en-us.pdf 

Sata ports are 0 - 4 but memory slots are 1 - 4.

Oh yeah. I checked the BIOS. Nothing to do there. I did replace the CMOS battery though, First time ever since 2011.

Memtest passed one cycle. Just as I was ready to power off, it picked up an inversion error.

So running memtest again with all six cores of my AMD enabled.

That's a good idea to run my sticks thru each slot, one at a time. I will do it,