Screen Display Issue after screen turns off

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
Have installed Elementary 5.1.2 Hera on my iMac CPU X7900 @2.80GHz, Advanced Micro Devices, Inc. [AMD/ATI] RV630/M76 4Gb memory, bought in 2007. Graphics works perfectly on booting up. However after the screen turns off when not in use, on waking it up (by say mouse movement or tapping on keyboard) the screen is very grainy, low res, not useable. Only solution is to hard power down and reboot.

Do I need to install a different graphics adapter driver? or ..?

### To Reproduce
As described above but of course, requires a 2007 iMac I assume to reproduce.
### Expected behavior
Screen resolution and clarity to remain rather than degrade to low res, fuzzy after waking up on screen turn off.

### Screenshots or screen recordings

### Logs

## Platform Information
As noted above
Please check what applies:
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- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version

## Additional context

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