I had the same thing and it turned out to be the solenoid was sticking. The next time it happens, quickly (but carefully since the engine may be running) lift the motor cover and tap the starter solenoid with a wrench or screw driver. If the starter disengages, it's the solenoid and you need to replace it. In case you don't know, the solenoid should be mounted on the back of the motor underneath the plastic shield. I would go ahead and remove the plastic shield prior to starting the motor again so that you are ready to "tap away". I'm pretty sure you'll find that's the problem.