Hi, I had this strange problem ever since I use FireFox. Changing to version 3.6 didn't help.
Having this frozen screen (for 1 sec) every couple of seconds was very annoying while watching videoclips on YouTube for example.
Even when using the plain browser functions like scrolling, it lagged.
Just today I found something that finally helped me to get rid of that problem completely.
Open FF ► navigate to about:config
► put into the filter: browser.cache .memory.enable
► set to false
by a double click ► restart FF