Any suggestions for a good stop to break up the drive between Orvieto and Positano? Hoping to spend an hour or two having lunch and stretching our legs somewhere en route.

Caserta? The 'Italian Versailles'?

https://www.google.com/search?q=case...=1600&bih=1075

Caserta is a almost at the destination, so I don't think it would make a good place to break the journey. Anyway, I saw a news program recently about severe maintenance problems at Caserta. I don't know how much that was exaggerated for journalistic effect.

I would suggest Tivoli, Frascati, or Palestrina as possible stopping points. They're all fairly near the route you would take (maximum 15 minutes' detour each way) and you could visit any of them before getting on the autostrada.

A bit further south, Montecassino has an amazing monastery, founded by St. Benedict, but almost completely reconstructed after being destroyed by Allied bombs during World War II. It's high on a cliff, in a splendid setting. There are also some Allied cemeteries in the immediately vicinity, especially a Polish cemetery. Many Poles served as volunteers with the Allied forces.

I did the same drive except I left from Citta della Pieve and it took about 4hrs, so from Orvieto should be a little bit shorter. All on freeways until you get to Napoli and then the fun part of the drive begins.
Caserta is over 3/4 of the way there, my advice is too push through and enjoy a wonderful lunch in Positano.

I second trizzy's suggestion - leave early enough that you can have a leisurely lunch by the sea in Positano!

The total trip is around 230 miles and really is doable in one go to be in Positano for lunch if you leave early enough. Most of the drive is on the autostrada until you get almost to Castellammare di Stabia and then on ordinary roads to the AC.

However, if you do want to see a bit more of the area, I'd recommend Frosinone as a stop. It's just off the autostrada, and is a good place to lunch and rest before getting back on the road. It's about 115 miles so about half way.