Welcome! Please make sure you post in the correct forum - in this case, you would be better off in the Mercury motor forum.
I had the same problem on my 1988 125hp Force outboard - would shift when attached to the muffs, but would cut out while in the water. I had to adjust what I believe is called the throttle linkage adjustment. Have someone SLOWLY shift into gear while you adjust the screw. On my engine, it is located on the right side of the block, behind and slightly lower than my bottom carburetor. Hope this helps!