Older 60,s merc 50 hp 4 cyl occasionally slipping in forward gear

[Sylvan 19.5]

I was out today motor ran fine until about 2 hrs into trip when it started revving high and little forward movement. I changed the prop today thinking it was a spun prop, still the problem occurs. When I shift into neutral then into reverse and back to forward it runs fine till the same thing happens maybe an hour later. Why would shift to reverse then to forward seem to temporarily fix the problem?