The muffler was the cure.

I put the mostly stock muffler on the engine and an APC 10 x 4 propeller. I broke the prefeered APC 9 x 4. Then I installed a normal O. S. brand needle valve assembly into my homemade venturi and the engine ran very well. i have a bunch of flights and it's running as good as it ever has. I have the new copy of the plane ready for paint. I may spray the base white later today. I found another brand of clear lacquer spray paint at Home Depot. I've been testing it on other planes to see how it reacts to engine exhaust. So far it's going well and I'm planning on using it as the top coat. I don't think the new plane will be ready for the El Dorado contest.