I favor the high mash temp theory. The one time I missed my FG by 5 points or more that was quite definitely the problem.
I monitor temp in the mash keggle using an analog Blichmann Brewmometer and a digital probe connected to a BCS-460. My mash profile is 150-152 F. The HERMS is under BCS-460 control, and is set to 153 F in summer, and slightly higher in winter. The returning wort temperature is monitored at the outlet of the HERMS coil. In my lightly insulated mash keggle I can maintain 151 F by running the recirculating pump 10 min on, and 10 min off.