My 1650 Lund Explorer/60 Merc used to do the same thing on calm days.I had to lift the motor 1 hole higher and use wedges between the motor and transom,solved the problem right away.The dealership said the motor was too low in the water and was causing drag.When i trimed it up to get any speed the front end would pop up and down like crazy,and when i trimed it down it was slow and plowing.There was no happy medium until i raised the motor and added the wedges.Also gained 4 MPH.Hope this helps.
