[Chinchi]

You are talking furbiting right?

It could be fungus. That would not explain why she bites her mate though.

Furbiting can be genetic (or at least tought by the parents), and it can be steriotypic behaviour.

If it's the last thing, it can be because of boardom, or sometimes they just seam to start, for no obivious reason.
Your girl sound like the last option.

I guess it could be some kind of stress reaction, because of some thing when she was with your parents.

But try looking up fungus. You can get some dust you can put in the sand.
If that dosen't help within a week or so (shouldn't be put in the sand for too long), your left with furbiting.

Furbiting dosen't hurt the animal in any way, and it is very hard to get them to stop.
The only thing you should make sure of, it no breeding on a furbiting animal, because the kits will get the bad habit.

I have one furbiter.
She began out of the blue one day. I have absolutly no idea what made her begin. She's with her sister, and has always been with her. The sister dosen't bite her fur at all...

 

[Chinchi]

Then get the male sterilised.
She's also beginning to get a bit old for breeding (yes, they can get much older, but 8 is still a lot )

The whole furbiting thing is a bit hard to find any good solid info about, mainly because it hasen't been studied that well.
Personally I don't think it's genetic, I think the kits simply learn it from the parents.
But it's always nice to list all the possibilities

Try out the fungus treatment.
It won't hurt the animals, and better safe than sorry.
A lot of vets don't know that much about what they are talking about, when they are dealing with rodents, and chinchillas are not that common, so it can be a problem.

Her mate dosen't suffer just because he bites his fur.
Sometimes they overgroom each other that way.

Oh yeah, you could try giving c-vitamin.
Sometimes when the fur gets kinda bad, and you can't find any logical reason, it's because they need some c-vitamins.
I'm not saying this is what's wrong with your animal - just throwing idears out there

And how often do you give calcium?
If you have the chance, try offering a cuttle bone (sepia).

 

[Chinchi]

The pellets should cover the vitamines on their own.
Never use the vitamines you need to put in water. They only work for a very short time (meaning your animal need to drink the whole bottle in a day or two to get vitamines), and it's too easy to give a wrong dosage because of this.
Now you could say that c-vitamin (the one normally put in water), can't really be overdosed, but it's still a wast to use it

If the cuttle bone dosen't work, at least they will have a fun time

You should be able to get the fungus powder from some online stores. Try out the ones you know.
I'm not sure where you are from, but I guess you are from the US, and I don't really know any good online shops from the US.

 

[Chinchi]

If you are interested, I just got the "recipe" for the fungus powder:
5 grams of sulphur
10 grams of babypowder

This should be used for 1 kilogram of chinchilladust