Have the wiring to the pitot head meggered for any intermittent earthing
I really wouldn't "megger" the wiring (unless using either an 80v or max 250v test set) , we are talking about 14-28v dc here, and if the fault is "intermittent" then you wont get any result anyway. If you did then is it the pitot or the wiring?
Forget all this talk of votage measuring at different rpm's etc etc, not practicle or correct. CB's trip out due to current overload, 10/1 the pitot head has failed as it is nothing more than a resistive device that provides a small amount of heat.