Why don't you re-write the articles to differentiate them from the other PLR articles?
I would never consider using PLR content on any site. While Google claims that there is no penalty applied to duplicate content, all the evidence proves otherwise. It may not technically be a penalty as Google describes it, but the net effect is the same.
I'm not surprised that you picked up some PR, but you are in danger so losing it on the inner pages if the site gets run through the duplicate content filter. My theory is that the filter is not part of the normal ranking algorithm, so duplicate content pages can initially rank well, but then get nailed when they get caught by the filter when it is run at a later date. When pages get hit, the toolbar PR turns to a gray bar and the pages get dumped into the supplemental index. The home page frequently escapes the PR penalty, but the inner pages rarely do.
I've seen this happen over and over with new affiliate marketing sites.
"It's inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians." -Henrik Ibsen