If the tank is new, then it could be stratified fuel. The new ethanol gasoline has a shelf life of approximately two months, and then it begins to "separate" in tiers. I'm not an expert on this, but I've been investigating a similar problem on my A-4, and read some helpful articles in Boat U.S. about the ethanol problem. Fuel stabilizer helps, as it extends the life of your fuel to a full year. This is important for sailboat owners, as we often just use a gallon or two at a time. Old fuel that has no stabilizer additive will begin to degrade much soon. I know I'm not explaining this all that well, as I'm in a hurry, but I think you can get my drift... If you refill after every sail in order to lessen the effects of condensation, then you are barely dilluting the fuel in the tank. A motorboat, on the other hand, can blast through 3/4 of a tank in a single afternoon, and then refill with new fuel. Stale ethanol fuel can lead to varnish build up, and it can also lead to symptoms similar to what you're experiencing.