I have a 1988 60hp Johnson motor with a new battery. Motor starts great and runs good at idle and low speed. When I go full throttle for about 30 secs the motor cuts out. When I try to restart the starter hesitates but I am usually able to start and then run at very low speed back to the dock. Once I have the boat in the drive way and run it on the ears it runs good. This started happening last year and I thought it was the battery but it obviously was not. Plugs are a nice brown color so they seem to be working fine. Any ideas?