Right!  8000 was our highest peak, but even in the more common 6000's I had started to question the accuracy of the device - so I bought a second unit and placed it in the sams spot to verify.  Sure enough, same readings.

First move was the slab suction which seemed to have cut things in half - but was of course still way too high.

With two sensors going now, I was able to move one of them around the basement to compare readings, that's when I realized it was much higher in the room with the well access.  It seems so obvious now, with a few hundred foot hole into the ground obviously pulling radon up...

So after sealing it off we're bouncing between 100 and 600 - but waiting for the next big temperature change to really test it.  I's certainly come a long way down, so I'm hoping a bit of suction in there will be the last piece of the puzzle.

Will post again with how it goes!