what temperatures are you working in? When the oil is too thick because of low temperatures, the friction increases in a such high amount that the engine isn't able to reach an adequate rpm to keep up running. If that rpm is only in range when you set full power, remember that low rpm and high manifold pressure produce heavy stress inside the engine, especially on the crankshaft. If your hardware is ok, you should look after the temperature in the weather settings.
Thanks a lot! That was it! My, my at my age I should have known better.
I really appreciate your help!