use Oauth, twitter are due to completely remove support for basic auth this month.
400 errors are often associated with hitting your API rate limit.
Go to this URL in a browser: http://twitter.com/account/rate_limit_status.xml
and see if you've hit your limit which is something like 150 requests per hour.
So for instance if you have timeline/mentions and DMs set at 1 min check interval, then you'd reliably blow that 150 limit every hour, and have to wait for the end of the hour to reset.