John

Something strange there as the Sensor switch regulates to 5.9v and the Li-Po does not get that low!
The voltage drop I see in 7-8 flights makes no difference to my range.
Are your servos musical? if you put the flaps to mid position does it change the range, if you disconnect each of the four wing servos one by one does it increase?

Only thing I can think of is more servo noise at the higher voltage using the sensor switch.

There are now at least 12 Flashes flying with my/your set up in the UK.

Dave