Running problem with 1991 ts6M Supra

HELP! 351 cu in PCM with 4 BBL Holley starts, idles and revs fine on trailer, but in the water it throttles in gear to about 2500 RPM and then coughs, misfires and flat-lines ( no acceleration ) and peaks at around 20 mph. It shifts fine between fwd and reverse at idle speed without hesitation. Carburetor has been replaced. Fuel pump delivers fine at 6.5 psig. Plugs and ignition wires have been replaced. Coil pack has been upgraded and distributor electronic sensor has been replaced. I'm thinking about scraping the wasted-spark ignition that it came with and converting to single coil with standard points distributor. Is there a simple problem here that someone else has experienced?

That looks like a PRO TEC system its probably the ECM . No parts are available for that system . I would install a Marine grade HEI distributor.