Well, I'm not sure how you bought the domain, but chances are it may not actually be a PR5. Especially if there's only a few backlinks.
Roughly you'll need a few PR5 & PR6 links if you truly have a PR5 domain. Check out
your backlinks and make sure that you actually have those type of links. If not,
you don't have a PR5.
Now, if it is a real PR5, you'll want to throw up a site quickly, get something up there
or Google will/could discount your PR.