Have you asked DELTA customer service?

They are very pleasant -- and keep sending me replacement cartridges. That isn't solving the problem and, in any case, I can't believe that I have gotten two sets of bad replacement cartridges in a row. No, I think there is something else going on . . .

Get a new faucet.

Faucet leaks can be caused by thermal expansion problems. If you are on a water system and you have a pressure reducing valve (PRV) there should be a thermal expansion tank installed proximate to the water heater.

If the thermal expansion tank loses it's air pressure charge or the bladder in it has a leak then pressure spikes can cause faucet leaks.

Best thing to do is get a pressure gauge with a tattletale needle and see if it registers spikes over 70 psi. If it does then check the tank.

You can learn about thermal expansion by clicking here

Thank you for the suggestion; that is something about which I had never thought. Our place does not have a thermal expansion tank and when I was talking about this with my wife she FINALLY got around to telling me that when she opens the hot water side of the faucet there is often what sounds like a spurt of air.

Hi, Did you replace the seats? They are spring loaded rubber cups located at the bottom of the valve. The cartridge rides on top of them. Use a small screwdriver or nail to remove them. The older seats have longer springs than the newer models. If you can't get a kit with the longer springs use the old ones.
Good Luck Woodbutcher