Only 64 volts across the bathroom heater? What gives?
I suspect that the hi-limit has been cycling because of insufficient air flow. (the blower fan is either plugged with dirt or the bearings in the motor are dry or both) The points on the hi-limit have arked to the point that they won't carry a load anymore.  You are just getting a carbon trail voltage reading.  Don't try to save this heat-fan-light. Replace it.  It is most likely a hazard. 
If you purchase and install a new unit, make sure that it goes on a 20-Amp dedicated circuit complete with 12-gauge wire. P.