Unfortunately, the hard drive is probably getting ready to die. If you don't have all of the iPod's content, I'd make a complete copy of it, including the hidden folders.
You may attempt to completely restore the iPod, but that won't extend the life of the hard drive.
By the way - you can always force it to restart by holding the menu+middle button. My iPod classic was lagging hard as well but a reboot fixed it right up.
I'd bet that manyof us are familiar with the scenario that DonFoy describes at Macsimum News: You're listening to your iPod when it inexplicably freezes up. No amount of buttonpushing will get it to do anything. Or, you plug it into your Mac and nothing happens. It doesn't show up on thedesktop...