My first question would be if you and your boss are using the same browser at home. I get (wildly) different results using OWA (Ex2003) using Netscape (7.2) and IE (7.0)
It was different enough with Netscape that I use IE for OWA (and only for OWA) at home.
I also recall that there are two authentication modes available in OWA.Ex2003. Using the ‘forms’ options for authentication wreaked havoc with performance because it didn’t play nice with my router and firewall configs at home. Flipping authentication made it behave better.
Since you’ve validated that it works reliably from outside the local network, it’s time to try to dig into what’s different between the systems…. if that’s a level of support your organization supports. I would assume that getting IT’s boss able to use OWA would qualify.
If she’s not using IE at home – and an up-to-date and patched version of IE – that’s where I’d start. If she’s not using windows at home, go with FireFox and the IE compatibility.