Managed Fusion IIS 6 caching 500 errors

I'm running a site which uses Managed Fusion to reverse proxy a third party site into a sub directory. Several times now the third party site has gone down and presented a 500 error. This in itself is not a problem, however, IIS seems to be caching the 500 error such that even when the third party site is back up, viewing the site through the reverse proxy still presents the error. The only way I've found of fixing this is by restarting the WWW service. Is there any way 开发者_JAVA百科to prevent this caching without damaging performance of my site?

We've decided to ditch the proxy and go with a subdomain for the external site.