MapServer version 4.4.2 application error WinXP

[Mark Howard]

I'm running MapServer version 4.4.2 on a Win XP Pro box. 
I have a javascript app that has an input who's src is the mapserver WMS
URL. 
I just update the src URL to pan, zoom ..... without refreshing the
entire page. 
After about 20 refreshes, I get no reply. This happens in IE and Mozilla
Firefox. Running Firefox, I get a dialog pop-up that says that there's
been an "application error - the instruction XXX at ZZZ could not be
read".

Anybody seen this before? 
 
TIA FOR ANY HELP!!!!!

Mark