This sounds a lot like Grumpy.  I had adopted her from a shelter in West
Branch, Iowa with a sister I called Doc.  (Doc was the exceptionally
brilliant one!)  Anyway, Grumpy and Doc came from an apparently extremely
abusive home - they were found on someone's front step in a shoebox.  Doc
started out as Bashful because she was scared of anything that moved when
I first got her.  Grumpy was Grumpy from the day I got her.  I tried the
shelter mom's nip training technique - biting her back on the scruff.
After a while, she stopped nipping so much.  But, out of the blue, she
would really latch on to my forearm or calf.  I tried the biting her, then
putting her away.  I tried the scruffing her for a certain amount of time
(usually a 10-count), saying no, explaining why I was scruffing her (mostly
for my own benefit - she usually looked at me like she didn't give a flip
why I was doing something that made her more mad) and then setting her
down.  She came back and bit me 6 times once after doing this, so I put her
away while Doc and her brother Dopey were out playing.  I had them boarded
at one house where the rule was if you bit, you got picked up for a certain
amount of time.  This gave her really mixed messages because for a while
she was biting to be put down.  After staying there, she started biting
to be picked up.  The longer she stayed in one place, the better she got.
Unfortunately, I was in a period in my life where I was moving every 6
months it seemed, so she never got the stability she really needed.  It was
after 3 years of this that I decided that my life wasn't going to sttle
down soon enough, so I surrendered my fuzzies so they could have a better
home than what I could offer them at the time.  I'm sorry I can't offer
more solid advice.  She never completely stopped nipping, although she
would go in cycles.  It was just very hard to trust her.  But, eventually,
with consistent training (and I know it gets so discouraging!), your fuzzy
will bite less, if not completely stop.
 
As for the getting bitten when you scruff her, I had that problem with
Grumpy too.  :-) What I learned to do was to use only the tips of my
fingers to to hold her, and then support her rump with my other hand.
That way, my knuckles weren't where she could get them.  Don't give up -
she'll come around!
 
Hope this helps,
Bethany
[Posted in FML issue 3312]