Macha,
Another thought - have you deleted the temporary files on your computer lately? I had a similar problem a few months back, and as well as clearing cookies, I used a program to delete my browsing history, temporary files etc (which were just taking up space doing nothing anyway!),..... In other words, I had a bit of a spring clean on Windows, and it's been fine ever since. I'm not technically minded enough to guarantee it'll work in your case, but the program is free - so you might want to give it a go!
Don't ask me how it worked, but it did! If you want me to point you in the direction of the program I used, I'd be happy to PM you the details.