You gave almost no information:
how many par2 blocks did the d/l have to begin with
where did you get the nzb for the d/l
have you identified the rar part(s) with the most errors
have you tried getting any of those mangled parts from another server
have you used a search server to see if there are any par2's that weren't in the nzb you used
have you confirmed that the nzb you have was an 'originally posted' nzb or was it something cobbled together by whoever
Tons of questions, tons of ways to figure out a solution.
I always look for an 'originally posted' nzb, if the posted uploaded one. Go from that first; even newzbin 'editors' miss lots of parts when then 'edit' the nzb; OR go to a site where the posted rar parts can be packed up into an nzb, and go from there.
Second, look for the worst mangled rar parts, and try to get more complete ones from a different server, like if you use Astraweb, try the EU/Astraweb server for 'fills', if not, try another decent server that sells block accounts, and use for 'fills'.
Lastly, if this turns out to be a continual problem, think about changing your main pay server. But you might look at WHERE the parts were posted (look at the message headers), it might be a wanky server somewhere, then again, it might be a wanky uploader, so check around on message boards (like this one) and list the full name of the d/l you are having trouble with, and see if anyone else as run into the problem.
Tons of stuff to do, I gather none of it has been done/attempted.