It sounds like your having what's called a soft seize. This happened to me and my Challenger, a few years back. I could run wot for about a half hour, then for no reason, it would start slowing down for no reason, till I dropped off plane. If I continued to try and hold throttle in it, it would die. But, if I idled it, I could make it back to the house.
On my 787cc motor, I found the problem to be part of my total loss cooling system. Between the cylinders, there is a water tube that comes to a "T". From there, it goes to the back of the boat to a calibrated fitting behind the water box. Every year, I'd take that line off and blow back through the motor, so figured everything was fine. But, I never blew back through to the outlet, of the calibrated fitting.
If your not flowing water correctly through the system as it's designed, you can stagnate your water to a point of expansion. The piston and rings close up the gap between your cylinder, slowing the engine. Then, when you slow down, the stagnate water is enough to cool it and contract, so you can continue to idle.
This may or may not be applicable to your problem. I'd recommend from here, to identify all your calibrated fittings (which are the elbows you see connected to the outlets of your motor, and blow through them. Because these elbows are calibrated to allow a certain water flow, they can be clogged by a piece of sand grit pretty easy. These calibrated fittings are equivalent to a thermostat. They ensure the motor stays flooded with water while in operation, and that the path of water circulation is complete.
Check them out, tell us what you find out.
NEVER MIND.. I just realized you have the 4tec motor. The info above only applies to the 2 stroke models. I'll have to re-visit my brain and re-hash your question.
O.K... Did you look at your old plugs? How did they look? Where they all colored (deposits) the same? I have a plug burn chart in Snipes Korner, which you can find by clicking the link below. Have you done a compression test on this motor? For the 4 stroke models, there are several things that could cause the symptoms to the problem your having.
Check plug color for consistent burn, then compression......
