craig 2016

Hi People,
Thankyou for looking at this.
I am new to this and am having problems with my latest edition st flux bullet.
Basically when driving the top speed is very intermittent.
It can run well at full speed for max 30 secs then reduce to 60% speed then either speed back up of cut out requiring reset which cures it immediately.
I have checked and tested with all the obvious ie battery safety cut out 3.0v, over heat protection.
I have tried a different esc and a different motor.( not ruling out the possibility that i have damaged both motors)
I am using 5 different good quality, new batteries one of which is 3s.
I have changed many parts inc wheel bearings slipper clutch and diff sets.
when the pinion is disconnected the motor runs consistently well for long periods.
it seems to happen as soon as there is any resistance on the motor/esc.
Any Ideas or suggestions will be appreciated
Thankyou Again