Ok, what kind of error messages are you getting?
I don't know the Mac utilities, but...
If you have broken rars, see if you can test to find out which rars are corrupt.
See if the corrupt files are the right size, if they are, then try the Rar utilitiies's repair function. If you are lucky, then there will be recovery information in the rars themselves - that would fix minor corruption.
Basically, if you have bad rars, and can't fix them, then try to find out which ones and download them again.