Bought a new Starweld 16' with a F70. First time out with a 15 pitch prop couldn't get above 3200. Changed the prop to an 11 and the RPM's went up and I was able to plane an get to suggested WOT (5800). The problem I'm having is after trolling for a while with that motor I can't get back up on plane. The RPM's stay at about 4200. It has done this just about every trip out. First thing after leaving the dock it runs great then after trolling for while I can't get my RPM's up to get back. Any suggestions?