"I think it may have to do with tonguing, because sometimes when starting to play a note, it feels like my tongue is cutting off the air flow and I don't get a clean attack every time, again, this does not happen all the time, but more often than I would like."
These are some of the things that could be possibly happening:
1. As you go higher in the register, the tongue is being lifted (arch tongue technique) and the tongue is filling up the oral cavity which shuts off the air. Possible solution? Play soft and work the corners by slowly pulling the mouthpiece away as you play. this will cause the corners (which should be working in conjunction with the tongue) to get working.
2. Your focus on your chin not being pushed up is possibly causing a problem in the opposite direction. It's being pulled down to the point that the lips quit vibrating(occilating). Possible solution? Don't pull the chin down.
3. Extreme pressure on the lips which can cause them to not buzz neatly. Possible solution? Soft playing (how soft is soft? I should be able to stand by you while you play and talk to someone without straining to hear or speak) play long tones but slowly begin to pull the mouthpiece from the lips. This will make the corners (which should be doing a lot of the work) activate.
Good luck and I hope this helps.