FF3 crashing... little help?

In the last few weeks Firefox's performance has gotten progressively worse for me. Cache was disabled before this (and still is). It's most noticeable on facebook, though it is repeatable (in differing time-frames) with all web browsing. After a few minutes online (started out about 30 mins, now it's down to 5 to 10), my processor will start running heavily; it'll bog down the whole machine (system monitor shows Firefox using all the available processing). The only option I have is to Ctrl-Alt-Backspace. It won't ever work itself out. It'll just eventually (another 5 or 10 mins later) crash. Digg does the same thing, but almost instantly.

Yesterday I fiddled with some about:config settings, but it didn't help, so I reverted back to the way they were before. I cleared all personal data, and that didn't help either. My ISP is S.L.O.W. Just barely can be called DSL -- I max out at about 100K on the download side, but typical speeds are more like 35 - 40K.

Does anyone recognize what's happening and have suggestions about what I can do to fix it? Thanks.