@DianeB I am sorry about your hen.
8 years old is getting up in age for a hen. Anything could be going on in her body to cause her crop to become slow to empty. The slower it gets, the more yeast builds up and hence the sour crop begins.
Many things can cause a crop to become slow from something as simple as too much sweet stuff, to a worm impaction in the gut, coccidiosis is famous for causing a slow crop, internal tumors that block the path of the intestines, liver disease, egg binding, just about anything that squeezes off the intestinal tract will slow the crop down.
Make sure she is kept wormed and has no cocci outbreaks. If you are seeing a lot of white urates in the poop along with diarrhea, the chances are good that she has an over load of cocci. I have had hens with cocci that will on and off empty the crop, but it does return. I get them on Corid for 5 to 7 days. If her backside has a lot of white urates stuck to her feathers, this can be another sign of cocci.
Get her wormed if you haven't done so. I use Safeguard Liquid Goat wormer. Kills pretty much everything. 1/2 ml for the standard sized bird, orally down the throat once a day for 3 to 5 days depending on worm over load severity. Repeat in 10 days.
Other than this, make sure she gets her greens. Greens are wonderful for the liver to help keep it clean. Go easy on anything sweet or illiimate it all together. (fruits, raisins, sweet breads).
And just keep up with all you are doing! 8 years old shows she gets the best of care.
Keep us posted!!