Generally the helmet connector takes most of the stress of the headset and tends to fail before any other component. If you've run the tests for bad mic and bad speakers (see the other FAQ questions in this category) you may have already determined a bad helmet connector. If you problem is intermittent, meaning it cuts in and out as you wiggle the helemt connector itself, then that's probably the culprit.
On our modular sets, you can get a replacement helmet connector. On our hardwired sets, you can send it here for repair or replacement.