i recently bought a 1986 bayliner capri with a 50hp force all in good shape the third time i took it out and after about 2hrs of riding i sat in the middle of the lake idleing for about 7or8 mins. then starting back in after a few moments the engine started to run for less than half speed after getting to the docks i cut it off then backed in the trailer and then it would not crank so after getting home i cleaned the spark plugs it fired right up so i bought new ones went back to lake and same problem i even put old plugs back in and still no help it will only run less than half speed can anyone help