Request Pacing

From PluginWiki

The plugin makes a lot of requests to Yahoo under the covers, many more than a human would do.  Yahoo has protection against spammers which means that if you make too many requests to Yahoo too rapidly, you may be blocked with a 999 error. 

To reduce the likelihood of this happening, you can tell the plugin to make requests more slowly.  This means your pages will take longer to load (some pages more than others, like the Pending Members page) - but you'll be blocked less often.

You configure this from Features->Even More Advanced Options:

If you see 999 errors in normal operation, you should experiment with this setting to find a value which works for you.  The left hand side, Fast, means don't put delays between requests.  The right hand side, Slow, means add delays.  So what you want is the value nearest to the left hand side which doesn't result in you being blocked with 999 errors.