Sounds like mine a while back. Ran great in neutral...couldn't get out of its own way in gear....

Two things: first, how long since the hull and prop were cleaned? Where I dock my boat, I'm only a mile from a sewage outfall, and while the plant is probably the most advanced in the whole US of A, it still makes for mighty healthy/vigorous marine growth. While my home brew anti-fouling works a treat, I can't get it to stick to the bronze of the prop. (The prop was new....) Last time the diver went down, the prop was like a basketball, though the rest of the hull just had easily-removed slime.

The other is that the ignition might not be advancing, as the centrifugal weights in the dizzy are rusted or stuck. Check for timing advance with a strobe light. Now there is no real timing mark, just a notch in the flywheel cover hole. Check to see that the cross-pin on the shaft advances with increasing throttle. In neutral, a lack of advance isn't of much importance performance-wise, but it is under load or in gear. Of course, if Prob. No. 1 exists, you're not likely to see #2 manifest itself.

Cheers