Help needed with Chevy 350

I need help trouble shooting a nasty engine problem. I have a 1990 Ski Supreme with a Merc 5.7l engine. I have been having trouble when the boat gets warmed up it seems to run worse. It looses power on the top end and also low end torque (i.e. pulling out of the hole). Last season I changed the wires, cap, points, plugs, and fuel filter. I was starting to think the problem was carb related and this year I changed the carb from the stock Quadrajet to a Holley (4175). The problem still presists. Tonight I took the boat out and timed how long it would take to start "booging down" and loosing top end. It took about 1 hour. It almost seems like the boats timing advance is off however, it runs good for the first hour so why would it start to have problems later? After the problem starts it kind of comes and goes but does get worse with time. I have used different types of gas and been staying away from BP (a problem in the past) I have checked the timing and it is where it should be. Also to note that the engine temp runs right around 140 deg. and never gets hot. The fuel line to the carb stays cool which eliminates the likelyhood of "vapor lock". I did purchase a coil but have not put it in yet. Any ideas? I need help fast!

Thanks!

this sounds like you are getting water in your hull the extra water will slosh to the back and seem like you are losing power. it doesnt take much water to do this. this would explain the time issue. do you have a auto bilge? when you are turning around to pick up a down rider watch your discharge. it is probably draining.

Thanks for the suggestion. The engine when it runs poor tends to sound "flat"...kind of like it wants to run but can't. Tonight the bidge was fairly dry...no wakeboarding...just running laps to test. Any other ideas?

sounds like a magnetic pick up failing, but you have points. The next time the engine starts to fail, spray the coil with a cool down spray, CO2 ,raido shack. This will tell you if the coil is bad.