hi , ive got a problem

... just been getting my tamiya m05 ready for the fryday night race meeting ( club racing ) ..
it has a novak ballistic 17.5t motor and a nosram matrix istc speedo , it has been quick and reliable ... until tonight when i decided to replace the gears and the diff .. but the motor was also removed and replaced again ..
but when it was being tested , the speedo started to cut out with just the blue light flashing ,
in the manual it says it could be thermal shut down ( but the motor and esc was cool ) ... the xps /power profiles could be to high , that was lowered , or it could be the gearing ..
when it goes , it goes as it always did , punchy , and good top end , but it just would cut out like a fail safe etc ....
could it be the sencor wire at fault , as my mate had a simular problem and it turned out to be the sencor wire .....
cheers shaun