Engine dies out when loading boat onto trailer

My 1992 60 hp mercury 2 stroke runs like a champ on the water for its age. The engine fires up on the first turn of the key every time and runs great all day but my last 3 times out only when I load the boat onto the trailer she dies out, almost like when the engine feels resistance. I then start the engine up again and give her gas and she quits again making me jump out of the boat and into the water and use the winch the rest of the way. Anyone ever come across this problem or know why this is happening. This never was an issue before. Thanks 

The engine is under a load when ur accelerating out on the water also By pushing you and the boat.