Thanks for all the replies.
On the hose and muffs it will not heat up probably because I always have enough pressure to feed the inlet and am reluctant to try and cut back on the hose to the point I see a difference. IOW I'm probably feeding plenty of water through with just hose pressure and can't simulate actual zero inlet pressure lake conditions.
I read on another site a guy speculating that the check balls wear out and when they don't seal at low rpm it doesn't force enough water through the engine. This seems backwards because they let water out and into the exhaust riser. Any definitive knowledge in this area?
I guess I could've gotten a bad pump but the symptoms have remained essentially unchanged through all my changes so that makes me unsuspicious of that.
Hadn't thought about the timing. It runs really well though so I doubt that it's retarded enough to cause problems although it's easy to check and I do still have a timing light.
Don't know about flaps in the exhaust. This system has large square risers off the top of the ex. manifolds and the exhaust goes in the water under the outdrive I believe.
Thanks again for the ideas. I'll mess with it this weekend and all things will be considered. My motto is "If you don't know what it is, you don't know what it isn't".