I have a 99 Johnson 60hp model J60PLEEB on a 20ft Lowe's Pontoon. It has started bogging out at full throttle but is getting worse. I can give it the throttle and it will be fine for 2 or 3 minutes at 6 1/2 rpms to 7 rpms then it will start bogging out until I can only run it at 2000rpm. It...