I have a 90 9.9 4 stroke Yamaha that I use as a kicker motor. Over the last 1 1/2 years it has been running intermittently.

- the motor will initially run great at trolling speed.
- after running approx. 10-20 minutes it begins to miss. When I slow down to an idle speed it sounds and runs fine - when I speed up it runs rough and sounds like it's running on one cylinder while continuing to miss.
- after fussing for 5 to 10 minutes it catches and will run correct the rest of the day at all speeds

Any ideas? I've taken it in to my usual place a few times for repair. Each time being told it's fixed when it isn't. The fuel filters are changed annually (sometimes twice), tune-up every year, etc.

Thanks for your help!