What software are you using to "un-rar" it? Have you tried unpacking it with an updated version of that software, or with another program altogether?
The few times I have had a corrupted RAR archive, it was caused by an outdated version of the software I was using, as the archive was created by a newer version of the software. I have seen anecdotal reports that using a different extractor altogether has met with some success.
I know there are extractors that purport to repair damaged archives, but I'm not seeing one that specifically mentions the more compressed RAR format.