A domain doesn't have PR. The website does. If you bought a website, and leave it hosted where it is, nothing will change. If you move it to a different hosting service it will get a new IP address. This may or may not effect the PR. Some folks say it will and others says it won't. My personal experience is that it will. I bought a PR3 site, moved it to my hosting company, and it is now a PR1, sometimes) Now that is only one site. Hopefully others will post and let us know how theirs went. Zeek
By keeping the content exactly the same and continuing to promote it, its not something you can really keep hold of, depends what the previous owner did to gain the pr.
Zeek, you case doesn't approve that a new hosting service affects PR. Though I'm not sure but I would guess the Pr decreasing is due to the decreasing of valuable backlinks to your site: the former owner removed some valuable backlinks. This is a popular trick in the domain market. Let me know if I'm wrong.
You should add content and start link building once you buy the domain name with Page rank otherwise the PR will be lost very fast.
Good point jackyork. Never thought of that. But I don't think that was the case. I bought it from a member here in DP. It was new and don't think it had any backlinks to begin with. Just a dumb Wordpress Adsense site. In fact, I forgot about it till the other day. Checked Google and I'll be darned if I didn't make a couple of bucks off it. Zeek
There are myths about PR and many people get easily fooled. Gone are the days when this was possible. Now google checks the domain registrant history and accordingly assigns page rank. Plus there are several other factors that effect the rankings.