The 4.3 Mercruiser is in a 21' Bayliner trophy and has performed excellent since new purchase. In July 2008 it began to run rough and lack power to accelerate and plane quickly. Of course a new tune-up was done, including cleaning carbureator, new distributor module, fuel pump pressure, fuel filter, new gasoline, new plugs (993's), new coil and compression test. The compression test was excellent - 170+ and consistent among cylanders. I have had it in the shop for repairs at least six times and each time I spend more money and the promise that it will now run okay. It still runs crappy and I am taking back to the shop on Monday to spend more money. I suspect the voltage on one of the electronic control modules is low - should be about 20,000 volts and heard one mechanic say it was about 6,000. But I am not the mechanic and the ones I hire don't seem to think it has any thing to do with that. Frankly, while I know quite a bit about engines and ignition systems, I do not have the testing equipment to check out electronic modules. Any advice you might give me would be appreciated.