I had this problem when I purchased my '97. Finally diagnosed it by letting it happen, then open the fuel cap, press in on the rubber gasket with your fingers and turn/remove the key. The cap will not close tightly, so have 1/2 tank of fuel or less. Mine ran fine with the cap open. Tho the vent tube looked fine, there was an obstruction several inches in that I couldn't see. Dirt daubers egg...
Really look for obstructions or a kink in the tank vent tube. If not there, this advise is worth what you paid for it.

Good luck. It's no fun not riding.