Well, your site is ranking for your keyword! and you are getting alot of traffic so why you care about your PR!!!?
PR is not having any relation with the SERP's. Don't worry and if you are doing well on SERP than you are going exact where you need to go.
Don't worry about PR, If your site has a great contents and good traffic, why are you bothered about PR. Now-a-days PR is meaning less