Cal 33-2 Vibration, shuddering, with Yanmar 3GM30 & Kanzaki cone transmission

Thought I'd post this for benefit of others with this drive system who may experience this problem. I've been through quite a problem solving process with my boat and its vibration. First thought it was due to Gori prop; removed prop (replaced with fixed prop), same problem. Thought it might be engine related; mechanic said engine running well. Not a bent shaft problem. Typically occurs only when engine is warm and only in forward gear. Dramatic shaking and shuddering. Can power through, but not good to do that! Finally isolated problem to the transmission. I disconnected the shift linkage cable from the lever attached to the Port side of the tranny. Then started and warmed the engine. I attached a small vice grips to the shift lever. With a warm engine, I manually shifted the linkage into forward. It shook and shuddered. Then, I put some pressure on the lever and held it. About 75% of the vibration and shaking disappeared. Shifted into reverse, no problem, even with no pressure. Tried the forward position several times, same results. The shop manual for the engine suggests that the forward cone can become worn and require linkage adjustment. The proper adjustment calls for moving some cover plates on the Port side of the tranny. Not easy to access. So, I took four turns on the adjustment nut at the tranny end of the shift linkage and reattached to the shift lever. Ran it through the gears and, again, about 75% of the vibration and shuddering has gone away. Again, no problem in reverse; only in forward. This was all done tied to the dock with the fixed, three blade prop. I plan to take the boat out today and check all of this out. Likely this is only a temporary fix and will require some transmission work. Travis