I have a 90 year old house with similar issues. The product you need are Plaster Washers. These are concave perforated disks. They are installed wit drywall or decking screws and anchor the plaster back to the lath. Your problem is that the plaster has unkeyed itself from the lath and is not attached where it is bowing or sagging. The place to get these is
I suggest buying more than you feel you'll need. I bought the 21 dozen set. Don't get the kits. They are an added expense, and you can get the screws at your favorite home center/hardware store. I needed both 1 5/8" screws and 2 1/2" screws. I chose to go with drywall screws. There are instructions on the site for how to install them. They work like a charm. I use a Dewalt 12 volt cordless and set my torque to 4 to start with so that I didn't sink the washer below the plaster surface. They end up slightly below the surface and the perforations really hold the patching plaster or joint compount extremely well.
Good luck. Saving plaster is nice in that it keeps with the character of the house. It is also a royal pain to remove as you'll be finding plaster dust in every nook and cranny of your house for years.