using external RSS feeds is always a problem. If the source from the site is down, or very slow. your tiki site really suffers.
It is best to try and not use them at all, or if you really must the only solution is to setup a cronjob for retrieval of them and then the tiki only displays what is in cache. Using this method also fixes the delay when its time for tiki to refresh the cache, which can also kill the site.
If you want the patch files and cronjob scripts, let me know and I'll email them.