2nd gear disengaging

[RJB1]

I recently purchased a 1947 Mercury 114 with 3 on the tree, V8 flathead. When cruising and not much torque on the engine it will drop out of 2nd and into neutral. This especially happens when depressing the clutch to slow down. My mechanic said it was probably just wear but it is annoying and would like to fix it. Any help appreciated.