You shoudl be able to isolate if it is the throttle linkage. push the button in on the handle to disengage the trany, then have someone look at the throttle body and see if there is a lag when moving the throttle to the forward postion. if it's slow you should be able to see it.
From my experience, it's probably a plug bad. It acts strangely when you have a bad plug. To test the plug, you can pull the plug coils one at a time at idle. If the plug is good, and the wire, you will hear a definite difference when you pull the plug. If you hear no difference when you pull the plug coil, it's a bad plug. Just don;t be standing in the water when you do this!
