Thanks for answering my questions. I really appreciate it.
Because this has been going on for some time now, my first thought would be that she's got some sort of regurgitation problem that is caused either by something out of whack in her digestive tract, or possibly from a doggy form of acid reflux.
The first thing I would do would be to call your vet on Monday and discuss it with him to see if he thinks there's anything in your pups past medical history that could be causing this to happen. I don't think you need to rush her to the emergency vet or anything because she is stable and is otherwise healthy and happy.
While you wait for the weekend to end to call your vet, try not allowing her to gobble her food (I know you said she only does it sometimes). Of couse, allow her to have access to water all day.
You can also try giving her her food spread out on a baking sheet, instead of in a bowl. If she's vomiting because she's eating too quickly, then spreading the food out will force her to slow down to eat.
Hopefully your vet will decide that this is just a little run-of-the-mill acid reflux that can be taken care of with a little Prilosec, but because she is a geriatric dog, it's best to have your vet take a peek...just to be safe.
I hope this helps.