I have a 92 Mercury 40hp 4 cyl that is hard to start as well as boggs and misfires above 2000rpms. At 4000 rpms it will run for about two minutes before shutting down completely. I have rebuilt carbs, replaced fuel lines, fuel pump, filter and installed new plugs. I used an inline spark tester which it appears to have spark on all cylinders. I am still leaning towards an electrical problem. Upon removing new plugs I noticed the top two and bottom plug are fouled while the third from top is not. Any suggestions would be greatly appreciated. Thanks in advance.