if the page is on subdomain of unrelated main domain affecting ranking badly

i have a domain about asp.net programming . i set up a sub domain about dentistry. the subdomain got a page rank 4 . but i find it very hard to rank even with the good pr . is subdomains is bad in ranking?? . or in other words should i register a new dentistry related new domain and transfer the pages to it or what??

 

This forum is on a subdomain yet ranks quite well.

 

think of subdomains as completely different websites! Your rankings will not be adversely affected!

 

straight to the point thanks.
but there is a last issue which is the completely unrelated main domain... do i need to change it ?? but the problem that i have a good pr that cost me a lot of efforts.or i have to go on with my old domain and try to make it rank??

 

it seems that i will stick to my old domain and try with it