First up try a bottle of Chevron Techron fuel additive. Directions on label.
How's the oil level?
What oil viscosity are you using. It's coming on to summer over there now, (I assume you are in NA) so use a 15W-40, as long as the temps don't drop below +10F. Delo 400 would be the ideal oil for this, it is a diesel oil, but rated for petrol (sorry, gas) as well, and will clean all the crap out of the motor.
Change the oil and filter at 3,000 mile intervals.
Let us know how you get on. Try the Techron first, and do a couple of long fast trips now and again.