Inconsistent encoding performance with Flash Video Encoder 8

I'm trying to suss out a weird technical problem I've been having over the past six weeks with Flash Video Encoder 8. Most of the time, it renders clips at a decent speed, taking about 75% of the actual playback length of the clip to render (45 seconds for a 1 minute clip). But every so often, the app renders at a breakneck speed, encoding at nearly 3x the playback speed (taking 20 seconds for a 1 minute clip). I'd like to figure out how to keep this faster rendering time so I can publish videos toour web site faster, but it's spotty.

I'm running the app on a 2.4Ghz IBM ThinkPad running Windows XP with 2GB RAM. Simultaneously, Norton AntiVirus is running on the machine with AutoProtect enabled, along with some of the default installation features of Office 2003. It's obviously capable of rendering clips really quickly, but I can't get it to consistently work as fast as it can. I'm wondering if a background service in Windows that's compromising the speed.

Anyone got any suggestions?