A code 400 usually results from either a corrupted packet which leaves the server unable to interpret the request, or from a request made using HTTP headers the server does not understand.
Why have you started getting dozens of them all of a sudden? --bad comm line, some flaky equipment, or someone probing for some specific server response. The only way to know for sure is to trap the packets arriving at the server and analyze them.
<added>Oh, and one more thing that may cause it, with Netscape in particular, if you recently added a form that retrieves values from a query string, make sure the URL is properly encoded i.e. no spaces, NN chokes on them and produces a "Bad Request" IE substitutes a %20 to fill in the space.</added>