Why does my iMac freeze since I upgraded to Lion? And how can I fix it?

Try running a Disk Permissions repair from Disk Utility. Also, try running a Repair Disk in Disk Utility. Look for any errors in the disk repair and post them here. Take Lion out of the equation, it sounds like a failing hard drive. It's possible that the upgrade brought out underlying symptoms.

When you say "clean install", did you download Lion again, or did you just use the recovery feature of Lion already installed on the iMac? I had problems on my 2010 MBP after upgrading to Lion. Apparently my Lion version was somehow flawed/corrupted. Unfortunately, this also meant that my recovery partition of Lion was corrupted. It was solved by going to a genius bar and the guy gave me a new copy. If you can get to a genius bar, or get your hands on a recovery stick that someone else has created, it might be worth a try.