Well, first off, what are you using as a controller? It sounds like the DMX signal is passing through it, so that's no problem there at least.
2nd, not to point fingers or place blame, but are you 100% sure beyond a shadow of any doubt that NOBODY else has tweaked with your scenes? I've learned that this is fairly common.
3rd: When using the fader assigned to that fixture's pan channel, does the light STILL not respond to PAN? It is properly responding on the right channels to everything else?
4th: How long is your DMX chain? Are you using DMX cable and NOT microphone cable? It may LOOK the same, but it does NOT behave the same even though it's wired the same.Just because XLR cabling IS working, it isn't what you SHOULD be using. Are you using a DMX terminator? Any DMX repeaters involved to bump up that signal?
5th: Does the place allow smoking? I'm not a smoker, but I have seen the damage smoking can do to electronics. However, even with 10 nights, smoking SHOULDN'T have much of an impact yet.
Having it 18 feet up in the air would tell me that it is highly unlikely that someone tweaked the addressing on the fixture itself. But, you never know. Some people are really MOTIVATED to screw with things. You never know what could happen!
Personally, I'd take the fixture down and test it in a controlled, or at least a more tighly controlled environment and see if the fixture is not behaving properly. And I understand that this is not the option you have due to logistics.
So, I'd directly access the fixture by going to the channels it is using directly. One by one I'd test functionality. Doesn't need to be "test values 0-255" sort of thorough, but moving the faders on the channels enogh to see if the light is responding.
Without going onto a manual with an X-move LED, my example would be with my Q-Spot 150's by Chauvet. I'd test pan, fine pan, tilt, fine tilt, gobo wheel, color wheel, gobo bounce, gobo spin.... other features. I have 9 channels, so it would take a few minutes to go through all the functions, say between 5 and 10 minutes at the most.
Regardless of how the light behaves in place during testing, I'd want to test it anyways going from controller directly to the fixture. If that doesn't work, then you know you have problem with either the light or the controller.(yeah, what if a fader or channel went bad...) It's best to have a second controller or something else to test with. Or change the channels the X-move LED uses and see if the problems follow the controller or the fixture.
It could very well be a fixture that has failed. Stuff happens. But you have to make sure, and you need to have direct acess to make sure. It's sounding like the fixture has a failure, but I like to be thorough and make certain. So, I'm suggesting an awful lot of things to try. It's probably NOT a controller failure, usually those exhibit other symptoms as well. I think the "fix" is to do a bit more testing to be sure, and then if it doesn't come out all happy happy, it's most likely time to return the unit to place of purchase or contact support for warranty service.
Oh, got any pictures? Just wondering what your set-up look like. Being nosy.