reverse gone when warm

I have a 1966 mustang with a C4 Transmission (not the one that came with the car when new) not sure what year it is. I replaced the modulator a few months back and it seemed to function fine. I drove it the other day and everything was fine when the engine and transmission was cold but when the transmission warms up I loose reverse. Fluid level is good. Have new lines, filters, radiator and fresh fluid. All forward and reverse gears work/shift fine when cold and forward gears work fine cold or warm but no reverse when warm. Whats up?:runaround: