AIS 6 crashes Firefox and PC when always sandboxed and slows IE8 a lot

As soon as AIS 6 installed on auto update crashed Firefox(latest version) and PC when always sandboxed and greatly slows IE 8. Seems to be better when using one off sandboxing.
When first purchased AIS 5 errors on sandboxing for some months–but eventually updates fixed this. Very disappointed to find AIS 6 similar on launch. Would definately be more secure if i could return to last ver AIS 5 --is this possible? Thanks
Thinkpad t41 1.6M 2GB memory XPPro
On reading post about 50% cpu usage with tagged browser —solved with using border instead- i tried swapping to border. This greatly improves situation but still freezes Firefox sometimes and has other problems such as not playing 4oD on YouTube–but 5 video OK.