Illustrator not responding or crashing imac

Illustrator 2020 keeps crashing my iMac (screen goes white and the iMac restarts on its own), it can't seem to open files 200Mb and up. I tried uninstalling and going back to 2019 but that's not helped either. Activity Monitor shows Ilustrator using 1.36GB when trying to start up. When it is running (trying to open a file designed in Illustrator 2020) even with no other programs running, it is barely making progress. 
I've cleaned the iMac with Cleanmymac and there's lots of hard drive space and I just can't work out what's making the whole iMac run slowly, especially when trying to run adobe programmes. Please please anyone help. 
It's a late 2012 iMac running Catalina.