Short story..........I carried my riding lawn mower to the shop a while back. Without the blades engaged it ran fine at high speed but everytime I engaged the blades it cut itself to half throttle. Thinking something was wrong with the deck I removed it, greased all fittings, went ahead and replaced a bad belt...but nothing really obviously bad. Anyway, put deck back on...same problem. So, after repairman charged me for fixing it he told me that one of the two sparkplugs was breaking down with a load on it and he replaced the inline fuel filter. I felt sorta foolish but like you, I thought the plugs were either alright or bad....no inbetween. I guess youre never too old to learn something new.