Big Dog, I can not recall all the details of your problem, but it seems like you have been battling it for awhile and, If I recall correctly, the motor inexplicably loses power under load, but idles and runs fine on the muffs.
I thought of another cause, the exhaust system contains a pair of anti-reversion valves called "water shutters" They go into the tops of the Y pipe, near the rubber couplers. They do wear out, and can break free and lodge downstream in the exhasut system, causing excessive back-pressure and power loss. It's something worth checking. To check, pull the elbows off the exhaust manifolds and look into the Y pipe. They should be readily apparent.
Here's a link with a bunch of different merc shutters, so you know what to look for:
http://images.google.com/imgres?imgu...%3Den%26sa%3DG