Never had a problem with the jack, the headphones on the other hand, were dropped from hip height into the grass and broke into about a billion pieces. (El cheapo twenty bucko from White's.) It might be a cold/iffy solder joint. I would try a different set of phones before faulting the MXT. Mine's had it's minor issues, but all in all..... a very good machine to hunt with.
HH as always,
Bri-