Man I would have the dealer by the balls by this time, it really sounds like fuel to me but they should have checked it all out, like inside the tank for debris, clogged lines, filters, injectors and pump,, Yes I would be talking to the owner by this time, and maybe dancing on his desk, or at least talking to the regional manager. They either haven't done a damn thing or don't know a damn thing, either way I feel for you, I had problems with a yellow tractor like that and got jerked around until I threatened to stop payment.