Sloane:

First thing I would check is if the transmitter in the equipment room hot to the touch. The fact that it works for so long then cuts out might be an indication that it's overheating and going into thermal shutdown. (I'm assuming they have thermal shutdown capability-but I don't know this for sure.) Could be a bad power supply, an internal short the transmitter or an over-driven input .

You have two transmitters, right?

Swap them and see if the problem follows the transmitter or stays with the location. You can swap receivers also but it sounds to me like your problem is either transmitter or something about the location.

Your equipment room and doors aren't full of metal are they? It could be you are shielding the transmitter from the receiver. If you swap transmitters and the problem stays in the equipment room then I would guess you have a shielding problem. If the problem moves to your sub then the transmitter is probably bad.

Hope this helps.