Compression should be up over 100, preferable per Merc serv. man, 120 min, all cylinders within 10-15% of each other. With your numbers I would question your gauge reliability and maybe your method of using it. Normally there is a difference between cylinders. At 50 psig, I doubt your engine would even crank, much less run.
Compression won't cause the problem mentioned. Check the engine for overheating....put your hand on the top of the block, above the top spark plug. It should be around 140F after running for that period of time. 140F is the Normal setting for US domestic hot water heaters to give you a reference point.
Problem could be stuck thermostat and or pop-off valve which, if installed takes over from the thermostat at about 2500 rpms to allow for additional cooling water at higher rpms. I know pop-offs are on larger engines, maybe not on 40 hp.
Then if it's not over temp then your fuel tank vent might not be working. When this condition occurs, what is the condition of the squeeze bulb? At WOT, it should be filled out but soft when squeezed indicating that some fuel is passing but not hard like you pump it up for starting where no fuel flows. If it is sucked flat your vent is not working, your fuel pump is sucking on the tank and creating a vacuum whereby no fuel flows.
Mark
