According to the Spektrum site the SR300 appears to be rated for up to 9.6V?

But without the regulator the amps being passed on to the servos would be higher, combined with more battery voltage sag under high servo loads that could lead to more voltage fluctuation which at the very least doesn't help. The regulator helps maintain a more consistent receiver voltage.

A glitch buster should help. And/or get a working 3100?