I have the same problem, with my ipod (80gb classic) freezing quite alot), I did some research and the solution is to hold both the menu button and the middle button down together, untill the screen goes black and restarts with the apple logo. It should be fine after this.
Spread the word, this was too hard to find out (it’s frozen like 4 or 5 times now and I only just discovered this).
If this fails, restore your ipod to factory settings and reimport your songs.