Hello all I have a 2008 Yamaha f75tlr 4 stroke. The last few times I have taken the boat out it idles rough and sometimes stalls, when I bring up the throttle its like it only runs on 3 cylinders but the 4th will come in and out creating a surge. once up on plane it runs good for the most part, to about 3/4 throttle anything past that and the motor about dies out and has no power until I bring the throttle back down to 3/4 or less. It has fresh ethanol free gas, the plugs were changed last season. any help would be appreciated.