Sometimes when I play, especially if my embouchure is a bit tired, the mouthpiece tends to slip upwards when playing high notes (which for me is anything above F4, more or less). And sometimes, it slips so far up that the rim reaches my lower lip, at which point the sound obiously stops.

Is this a common problem? It only happens on high notes, as mentioned. I don't think I use too little mouthpiece pressure, as I always get a red ring on my upper lip, usually after just 10 minutes or so. So if anything, I think maybe my mouthpiece pressure is too high.

I think my embouchure is quite wet, but not extremely wet. And my mouthpiece placement is quite traditional, 2/3 above and 1/3 below (when it's not slipping, that is). Mouthpiece is a Yamaha 48. I've tried bigger mouthpieces, but found it hard to get a focused sound with anything bigger.

Have a Skype lesson with Doug Elliott and yes I know this is a standard response, but no one can really advise and help if they can't see or hear you?

Or

If you have a teacher then talk to him/her as they will know your playing

My guess, worth 0¢, is that your placement is quite a bit higher (or should be) than you think it is, and as you get tired it's trying to move to a place on your anchor lip (your upper lip) where it is secure. On such a narrow cup as a 48, you physically can't move the mouthpiece to the ideal placement without it moving off your upper lip. The mouthpiece shouldn't be moving all over your face as you play - you jaw should be moving a bit and your lips should be shifting slightly over your teeth.

My guess is that a much larger cup width would feel a lot better and give enough room for your embouchure to work naturally. You have tried "larger" mouthpieces, but usually that means that the mouthpiece cup depth gets deeper as it gets wider. You need a wider, shallow mouthpiece.

Maybe try a Doug Elliott setup, sized between a 103 and 106 (no way to tell) with a C, C+, or D cup and matching shank.

Or, try the Griego Alessi 1A or 3A small shank, which is a similar concept.

Try everything. Don't take my word for it.

Good luck!

I'm not even going to guess without seeing it.

Contact me if you're interested.