25 Yamaha 2-Stroke Problems

[duckbuckhunter36]

Motor is 25 Yamaha SJ series (1986). Ran great last year on the water, this year it has given me some major headaches. I launched the boat, ran down the canal for about 3-5 minutes and motor slowly starts tapering down until it dies. When started again, it rev's up to max RPM. This has happened 3 times, 3 different trips. I brought it to be fixed, and was told the needle in carb was stuck. The last time, I said screw it, started it in gear, and it took off, but didn't sound like it was running to the fullest of its capability. Any help, ideas or guidance would greatly be appreciated.