Sorry to hear about your problems. In any situation like this you have to work throught the whole chain of stuff and carefully try to isolate the problem. I would not be 100% sure that the problem is with the 744. If you are running the Soundfield ST350 as it says in your byline, I would not really dismiss that one without first checking carefully. The Soundfield mic has a bit of electronics and there are also a bunch of cables to look at. So here are some of my, very unscientific, ideas for testing where the problem could be.
1 - put together and connect everything as on a concert recording. Put a few extra blankets around the recorder and Soundfield device -- this will isolate to make the stuff a bit hotter, this might be a heat related problem. So allow the bag to get really hot -- the 744T generates quite a bit of heat so that should not be a problem. Just maybe this heat will make the 744 misbehave, but it could also make the Soundfield control unit to misbehave.
2 - while keeping on recording, check that everything works. Juggle the bag and jump around it to see if could be a cable that is the problem.
3 - drink beer around the microphone to see if it could be sensitive to that (not quite serious this one, but it shows how to try to really recreate the problem at home by recreating the on-location environment). Remember that microphones are sensitive to heat and humidity. One way to test is use the shower at home. Let it run with hot water and fill the shower room with steam. Try the microphone in that room for half an hour. (DonĀ“t shower the microphone, that is not the objective here). Afterwards let it dry for a few days beeing in an aired normal room.
End of ideas right now. But unless you can recreate the problem it will be very difficult to find the solution.
Gunnar