My 1966 Evinrude Fisherman was new to me 3 years ago. It sat for several years before I got it. First year is ran OK but the RPMs and power were a bit unpredictable. Last summer it did not get used at all. I'm ready to use it alot this summer. Now it is very unpredictable in that it will run strong for about a minute or less and then slow down to a slow speed, and some times stop altogether, with throttle full on. It might rev back up after slowing down but usually stays slow. Gas tank supply lines are old but seem not to be leaking. No cracks in the other lines. Any suggestions on how to begin the diagnosis and repair of this problem?