Hi Robert, what OS are you running?

If the entire machine freezes instead of just one program crashing that is pretty likely to be a hardware problem.

I did a quick search on "iMac 2011 freezing" and saw quite a lot of results, like this:
https://forums.macrumors.com/threads/2011-imac-freezing.1173980/

Does your system have the original hard drive it came with?

Also from your description that kind of thing with "entire machine" problems after a certain period of heavier use could be from overheating. An older machine can accumulate dust in it and prevent its cooling system from working as well as it originally did. It would probably be a good idea to try and clean dust out of it like this:
https://www.youtube.com/watch?v=TwASThJaztQ

- Michael