Problems w/ Yamaha OX66 250HP

[TwoWisemen]

I've taken alot of information from this forum as I've spent the last year rebuilding my '76 23 CC. Now I have mechanical issues. I've been back on the water for two weeks now and I've noticed an engine irregularity that I have played with and it is 100% the same everytime.

I have a 1998 Yamaha OX66 250. Cranks & idles perfectly. I bump it into gear and it runs rough, like it's misfiring. I slowly give more throttle and it immediately clears up from 500 rpm to 1400. Then it begins to "misfire" again. As I slowly give more throttle, fuel consumption triples from 1400-1700+/- rpm as it feels rougher and rougher. Then all of a sudden, the motor shoots to 3200 rpm and runs smooth all the way to WOT. As I come back down off of the throttle, the smae thing happens in reverse. Nothing between 3200 and 16-1700 rpm and then it's rough back down to 1400 rpm. Also, the first time I took it out, WOT was 5800 rpm. The second and all times since, with less weight, WOT is 5100 rpm. O2 sensor? The motor has always been run w/ Ring Free or Sea Foam. Any help or ideas are much appreciated.