Check for water in fuel...it's the most common of fuel related problem. To check, pour the contents of the tank into a transluscent container, e.g. a pastic milk or Poland Spring container. If the contents looks like salad dressing (separating as oil and vinegar do), you've found the source of your problem. We'll go from there on resolving it.
If not let us know, so we know that one is ruled out.