Possibly junk floating around in the gas tank. As your going up a hill, the stuff floats to the fuel pick up line and clogs it. Once it has a few minutes to settle down, it goes back to the bottom of the tank.
In reality though, with a 23 year old vehicle, it could be one thing or a combo of several small things....dirty fuel tank, cracked fuel line, clogged fuel filter, and it all probably needs to be serviced eventually.
Start with the cheap fixes first, fuel filter and SeaFoam and work your way thru the more expensive fixes