Rob, it is a battery supply problem, I agree..
I might try 2 cell Lipo pack with an external regulator (LM317T) as suggested by Kermit, to keep the supply steady at 5.5 volts. I love to experiment, but the cost of parts is killin' me right now..$50 this week and since I am currently unemployed, that hurts a bit. Still have to change the initial curve in the PIC16f628 I am using..my 20cc motor with the 15" prop can take it but the 2.4 cu. inch chainsaw motor I converted doesn't like the steps in timing..