Pressure Washers

I just bought a pressure washer it is a 3100 psi with electric start. when using the engine runs fine but when you let off on the spray gun the motor shakes real bad foe a couple of seconds and then goes back to running normally. why is it doing this i had a 3000 psi washer i bought from sears and it didn't do that I had it for 8 or ten years and never had a problem with it until i moved from Fla to tn and i didn't know you had to winterize it the pump froze that is why i bought this one any help would be greatly appreciated. thanx Glenn