2009 90 hp Honda Surging

2009 90 Honda on Carolina skiff. Boat did sit for about a year. Just Took out and engine would surge like I was giving throttle up and down. Also would idle rough. Took to independent marine mechanic who I trust He lake tested then pulled injectors and ultrasonic cleaned them, changed plugs, and cleaned filters vs replacing since he would have to order them. On 2nd lake test, rough idle gone, runs great up to approx 1500 rpms then starts surging up to about 3500 rpms then will run fine above that until coming back down to about 3500 then it's like hitting a wall and starts surging. He doesn't have access to computer for Honda. Says fuel pressure good. Says we need to get to a dealer who can put on computer. Not a lot of dealers in my area. Thoughts or suggestions appreciated. Best regards.

Firstly I would replace all filters including the high pressure filter in the vapor separator (most important since it sat for so long). If when you have done that and the problem persists there is a good chance your bad fuel and rough running has contaminated the oxygen sensor. To verify this it needs to be graphed and monitored on Dr H diagnostics or take a chance and replace it. From what you're describing , the O2 sensor is about the only thing that can cause this issue, but do not replace until all the filters have been replaced.You can try soaking the O2 sensor in throttle body cleaner overnight and trying it but usually once it's damaged it will need replacement