[Serum]

It sounds to me like a damaged capacitor on your speedo..

The pulses that the controller takes from the batteries are deadly (temporary ) for the voltage of the batteries. (batteries simple can't stand any high frequency pulse load) .A capacitor absorbs those pulses and makes it a nice and easy on the batts clean amp draw. When a capacitor dies, it is no longer able to absorb those pulses, and may cause in what you are suffering.

However it also might have got something to do with the BEC on your speedo. you could exclude both things;

one by adding a capacitor to your circuit and two to use a seperate receiver pack.