Windows 7: Mozilla Firefox 4 Beta 6 Cache Issues?

I found something rather odd about Firefox 4 Beta 6. By default, it has a max cache setting of 250 MB. When left on this setting, after a while the browser becomes extremely unstable and slow on my computer. I think it is the cache doing this because the problem stops as soon as I clear it. To try to prevent this from continuing, I greatly reduced the cache maximum from default to 25 MB. Here are the system specifications for the system on which this has been occurring:

Compal NBLB2 Motherboard (PM55 laptop)
Intel Core i7 740 Quad-core Mobile CPU
8 GB 1333 MHz DDR3 (4GB x2)
500GB HDD (7200 RPM, 16MB cache)



After this change, I've noticed much better performance. This did not happen to me in version 3.6, but I really like the Beta. I'm just wondering what might be causing this.