Early iMac freezes but mouse still moves?

Hey everyone, I'm hoping somebody might be able to help me!

I have an iMac mid 2007 model and every so often the main desktop will freeze up on me but the mouse still works/time still changes but click on mouse doesn't work nor does keyboard. Last night it happened around four times but this evening I've had to do about 11 - 12 hard reboots. I was using yosemite (which this was still happening) so I thought if I update to El Capitan this might help but it still happens.

I have been running smc fan control to try and help combat this after trying to research this issue but to no luck.

It can be pretty instant after booting up or just happens at random times. One thing I have noticed this evening is that it hasn't asked me to sign into iCloud which it normally does. I don't know if that would be related somehow as it still happens when it does.

Any help people?