Fuzzy graphics/screen after suspend iMAc 2006

I installed 18.04.03 on a fresh SSD in a 2006 iMac with AMD RV630 Radeon HD 2600 video. The install was relatively trouble free - once the wifi card drivers were installed - except the display is messed up by suspend; it comes back, but it looks weird, like only half the pixels are being used, and it shimmers.

I tried to install the custom driver, but it doesn't like 18.04.03 (too new).
I tried turning off nouveau graphics in grub, that caused a black screen.

It would be great to not have to fully shut down; even with an SSD it takes 37s to boot.
Has anyone fixed this/have any other suggestions?