84 mercury wont run at top speed

by bill joe
(cuba, mo.)

have a 84 mercury on a 84 skeeter boat runs great for 1 to 2 miles and them starts to slow down and want run above 1500 rpms try to thottle it up it well try to die we took back where we bought it they put new swicht box took back to lake still do same thing put new stator still no change new fuel tank new fuel line and bulb no change please help