need help with fuel or ign. problem

[briefrecess]

I have a 1989 WEBBCRAFT with 454 mag and Mercruiser thunderbolt ignition. After it warms up it starts to cut out or loose power. It is not gradual loss but jerky. If I then bring it down to idle, it will usually run fine and may take off again. I can run it out of gear at full throttle without a problem; it is when it is in gear, under a load and the heat has "soaked in". After the problem developed, I blew the engine from an unrelated problem. I replaced the engine with a 502 Mag including an electric fuel pump and removed the anti-siphoning valve from the fuel tank. So, I do not think it is fuel delivery but electrical in nature. Additionally, I have replaced the cap & rotor, the coil, plugs & wires. Those were the cheap electrical parts. I have looked at replacing the ignition enhancer and the module under the rotor but these are expensive parts and I do not want to keep throwing parts at it. Does anyone have suggestions or had this problem before?

[AMER3]

my 86 scarab was doing the same think put cap rotor and wire on it still ran bad . trun out to be the pick up under the rotor runs great now.

[cabin fever]

Might try replacing your fuel filter before any exspensive IGN. parts.

[BajaJEFF]

I had the same thing on a 87 350 merc and it was the magnitic pickup under the roter. I found it by checking the timing when it started to act up, the timing was bouncing all over the place. I tore into the dist. and found the actual magnet was loose.

[Elrod62]

Had the same problem on my 98 29' outlaw 454s w/gm delco distributors one of the motors had a bad ignition module, i just swapped it with the other then it ran fine and the other ran like crap, so I replaced both $50 ea.