Dear Brains Trust,
Last two sails in the gusts, my board (fanatic hawk 120lt) has been doing a mono when powered up. The nose has been coming right up about 1 metre and it feels as if only the fin is in the water. My mast track is in the middle and the sail downhauled to recommened setting. I have been using a 6.2m RS Slalom in 20 to 25kts and a 7.0m RS Slalom in 17 to 22 kts. I'm 90kg and with the 6.2 I was using a 38cm freeride fin and a 46cm freeride with 7.0m. Most times I have been able to tame the beast and get Her back on the water, but had a decent stack yesterday with the 7.0m. Thoughts?

Could be that the fin is too big for the sail. Try the 38cm with the 7m. The 38cm fin in my 118 lt board was good until the 8.5 sail.
Could also move the mast further forward on the track.

mast foot a bit more forward

Try movng your weight more forward & sink into the harness this will give you way more mast foot pressure

Too much Christmas dinner!

Sometimes it is a commitment issue, sheet in, hold on and go for it, if you back off the nose will lift resulting in a nose lifting smash.

Thanks for most peoples input except for Jeff (who has changed over to kiting so comments are not relevant). Not sure if it is commitment, as everytime it has happened I have been fully sheeted in and speed was on the increase, when the nose started to lift. Thinking I may change my front footstrap slightly to angle me slightly more forward and move the mast track forward a touch. I have changed to waist/seat harness combo of late and shorter harness lines, because of the slightly higher hook. May have to change back to seat harness to keep the speed required to pass kiters.

I have a hawk 120 and expect the 38 fin you're refer to is the one that came with the board. In the years I've owned it I've almost never tail-walked it. Unless you're super light, I don't think it's the problem.

More likely mast foot pressure - possibly caused by the difference in stance between seat and waist harness. While arguably not good technique, seat lets you get away with 'hunching' down to the harness and keeps the nose down through pressure. Waist forces you upright, and hunching won't significantly increase down-force. You may need to look at adjusting your sailing stance with the waist harness.

Dean, just drop the boom head 50mm from where you usually set it. problem solved.

Maybe try the boom height thing but also try moving straps back and out to get more leverage on fin.
Get a good quality 32-33cm fin.
My Black Project type R 32.5 is my big sail fin. At 93kg 7m sail, 98litre board.