I agree with the above comments, the first thing you should do is sort out your temps, and provide a paper substrate.
Without having all the details it is a little hard, but if you have suspected mites, treated for it, then seen improvement, and now you are noticing the behavior again, it is possible that the mite problem has reccured.. You need to do several treatments to break the mites reproductive cycle. (If that is infact the problem.. Mite treatments are toxic by nature, and best avoided unless necessary)
Another reason for gapeing (holding mouth open) could be RI. I would suspect other thing first, going by what you have said, but it is worth checking for any poopping or crackling noises. You can also often feel it in the top third of the snake.
To do this, I reccomend loosely holding the snake up, with its head in the vacinity of your ear. Dont fully restrain it. You will need to listen, in a quiet area, for 5-10 mins to be sure, as they will often mask it for a few mins.
Without more info, it is hard to be any more specific.. If your temps turn out to be ok, and you see no signs of RI, would it be possible for you to have someone with experience check the snake for mites or any other probs, and run an eye over your set up??