Geoff - I would check / replace the governor assembly. This is a mechanical part that is accessed from outside the transmission (there will be a tin cover rear of the modulator valve on the passenger side of the transmission). Pop the cover off and the assembly slides out. It will have weights on the outer end and a nylon drive gear on the inner end, sometimes the gear will self destruct ... Very easy to replace and usually cost about $50-$60. I hope this helps ! Randy