2002 90 2 stroke wot power loss

I have a 2002 yamaha 90 2 stroke that is losing power at wot. When I got the boat new, it didn't have a tach but would sit you down when accelerating and ran really well with a 17p yamaha prop. While being stationed out west it began to show a noticeable loss of power at wot and ever once in a while while running the bulb would lose pressure and have to be pumped back up. Sometimes only once in several trips and sometimes a couple times in a single trip. I chalked it up to the altitude and propped down to a 15p hustler aluminum. A tach was also installed at this time. WOT rpm's were only 4700 with the 15p prop. Now the boat is back in eastern NC and still doing the same thing. RPM's are down to 4400 WOT. Compression is around 118 in all three cylinders. The carbs have been cleaned. The fuel filter and pump have been replaced. These showed no improvement so I moved to the fuel tank side. The bulb, hoses, filler tube, and anti-siphon valve were replaced today. It showed no improvement in rpm's but the bulb didn't lose pressure. That doesn't mean that problem isn't fixed because sometimes I can go a month or so before it does it again. It probably wouldn't be a bad idea to take it in and have the timing adjusted but other than this, what else could be causing this that I haven't looked at?