I have a Mercruiser 233 with the Ford 351. Sometimes when I start the engine the starter will not disengage after it starts even if I turn the key back to the off position the engine is still running.It will stop after I disconnect the battery cable. I have replaced the solenoid three times, but it seems to do the same thing after a few trips out.I thought maybe it was the ignition switch,but that did not fix the problem.Any ideas?