What security software are you running? Apart from ZoneAlarm, Spybot S&D's teatimer/immunisation can cause issues.

I saw this:

I really like it....(add-ons are great)
You can have too many and they might not all play nice with each other or other applications you have running.

Have you looked at your RAM and CPU usage when this is happening?

I guess the first thing I would do is create a new profile with only basic add-ins if any. NoScript and AdBlock are the two I generally use. If that makes a difference then it is an add-in issue of some sort.

What are your system specs and what OS are you using?