Possibly water in the tank, or a plugged fuel filter - both will do this sort of thing.
First things first, a $2 bottle of HEET will take care of the water by mixing it with alcohol and letting the engine run ‘roughly’ on this mixture - it will get the water out.
The fuel filter could certainly be a contributing factor as well.
I would NOT advise putting the various additives that dissolve the ‘gunk’ sitting in your fuel tank. That ‘gunk’ is sticking to the sides of your tank and ain’t bothering nuthin. If you add the ‘gunk’ disolver; prepare to have all that gunk hit your fuel lines and make your life a living hell. Don’t kick a sleeping dog; m’kay?
The HEET and fuel filter would be my first plans of attack.
Best post, Hodar...sounds like a fuel delivery issue to me.