Engine dies only in gear and when it gets up to temp
 

I haven't been on here in years as life has kept me from the boat, which I'm getting ready to sell. Took it out for one last spin and and had the engine die. This has happened in the past but seems worse.

Engine starts just fine, but once I get under way after 20 min or so it bogs down and dies, unless I put it in neutral where it will keep running. It will start back up immediately unless I put it back in gear. The whole thing seems heat related.

I rebuilt the engine about 100 hours ago.
Tank vent is clear, and above the engine by a couple feet.
Still has the mechanical fuel pump which was also rebuilt.
Carb seems clean.
Filters are fine.
Fresh nonethanol fuel, even added a bit of aviation fuel.
New plugs and coil
has EI ignition with good cap and rotor

I'm stumped

I'll take a stab at it. Sounds like drive shaft binding after a little while or a fowled prop with a plastic bag, dock line etc. After checking the prop try tying up at a dock, gently run the engine in forward until it dies. Immediately find neutral and see if you can rotate the drive shaft by hand. If this is too difficult to manage sometimes shafts that are "free" slowly rotate in neutral due to oil friction the plates [longer story] or rotate in neutral while sailing. After twenty minutes of motoring the idea is to determine if the drive shaft/prop is causing the problem. The other heat related cause could be a coil that is overheating and or with some friction in the distributor advance weights. I would eliminate these two issues first before moving into considering the carb that has not been regularly used for a long time.

Stupid question: but when it dies like this, can you spin the shfat by hand while in neutral? If it does not, you know it is 100% shaft related, and can exclude the rest

-- next time your sail, or tied at the dock, run it until it "dies" again, and put your hand on the stuffing box: it is is hot, you may have your culprit. If you had something tied in your prop, you would probably have the issue as soon as you engage the clutch.
-- as already mentioned, an overheating coil could be the issue; I had that in the past, and had to install a "heat shield", between the coil and the exhaust (it was my fault, I had moved the coil 2 inches closer to it...)