1997 Honda Accord RPM Tachometer bounces/engine then hesita

I have a 1997 Honda Accord with almost 200,000 on it. When I drive it for more than 20 miles at a time the RPM tachometer stats to bounce up and down and then the engine starts to hesitate. I can continue driving but the tach continues to bounce and the hesitation also continues. If I stop and shut the car off and let it cool, then it is fine again until the engine warms (again after about 20 miles of driving). Do you have any idea on what this could be?? Thanks!!