Transmission won't shift into reverse when hot?

Trans shifts fine when trans fluid cold. It shifts fine in forward gears when hot and at any speed. However, after driving for about 10 miles or so, when the trans fluid is hot (normal operating temp), when shift lever placed in reverse, no movement. In fact, if on a slight incline, with the shift lever in reverse, the car will roll forward as if it is in neutral. I had an independent MB shop change fluid and fluid filter, drove for a few days, no change, they then flushed the trans fluid again under pressure (did not remove the transmission pan), replaced shift rods bushings and the modulator, and send me on my way. There was some improvement, but the car still won't go in reverse after the trans fluid gets hot. After the car sits for a few hoursd, then I can back up the car, again until the fluid gets hot, then the car won't go backwards. The shop said I need trans rebuild at about $2500.00. This is after they already took about $700.00 from me.
Question: is ther any other alternative other then rebuild? Also, I hear some "buzzing" once in a while from the floorboards, like trans fluid pumping?