I have a 60hp Johnson that starts and idles flawlessly, and I can wind it out for a while with no problem. After I have run a gallon or two through it, it starts losing power. When I wind it out, it starts dying. I can back off of it for a few seconds, and then wind it out again for 30 seconds before it starts dying again. When this starts happening, I have to make a beeline for the ramp because it gets progressively worse until I lose almost all power. The motor has new spark plugs, a new gas tank, and a new fueline and bulb. Any suggestions?