3 cyl 40merc 98 model problem?

i can be motoring along all day no problems on calm water until another boat creates a bow wave,as i jump the wave and hit the water again i seem to lose power too one of my cyls i can be sitting on 30mph and she drops out to about 10mph on just under full throtle, so i back off and limp back to the ramp if i choke the motor briefly it fires up again, then back to two cyls,on the side of the carbys is a drain screw when i undo them and prime fuel through them replace the screws the motor runs perfect again,i dont know why. ANY HELP much appreciated