Took the boat out Sunday and when I was in gear, my 90 hp Merc would only get up to 4000 rpm and every couple of seconds there was a bit of a jolt, kind of like a miss fire or something similar. I went back to the dock and pulled the plugs, all of which were very clean and not very hot. Took off again and it opened up like there was no issue and ran like that for around 20 seconds and then rpm and speed dropped and I could not get it up to 4000. It ran steadily at that speed and did not have any intermittant miss so I motored back to the dock. The motor idled fine and would rev to top rpm when in neutral. So it seems that when the motor is under load, it will not perform. I am going to take it to a mechanic for a diagnosis but wondered if anyone had experienced anything similar. The motor is a '87 Merc 90 so not new by any means but has run great until now. I think it is not fuel related as it it runs cleanly when in neutral. So that leaves electrical or gear related. Any thoughts?