Hi Rob -
I tried digging around myself and played with PS to see if I could replicate the issue. So far no easy answer. I did come up with some info about graphic card issues that were solved with updates.
You mentioned you're on Windows - what version?
Hi Rob -
I know and understand how frustrating this can be - takes time and patience which can quickly run dry. I just updated to Windows 10 from 7 and I'm having troubles with the landscape myself.
I found some comments on this same issue using Win10 and Photoshop 2020. Seems one way some people solved it was to update their graphic drivers, uninstall, and reinstall Photoshop.
I'm also reaching out to @thebestcpu (John) to see if he has any opinion. He sometimes jumps in on technical issues.
I'll keep poking around myself to see if I can find anything further.
Saw the reach out from @JeffK
I think the starting points Jeff gave are great.
Intermittent types of failure are some of the hardest problems to debug
Here is a link to a pretty complete list of troubleshooting techniques for Photoshop. Going through all fo the options might keep you busy for a bit: https://helpx.adobe.com/photoshop/kb/basic-troubleshooting.html
Never forget the rule that when things start to go wrong, always try resetting PS Preferences. The Preferences file can get corrupted (typically a software bug) and you can spend a lot of time wondering what is going wrong when all it takes is to reset them (probably not you issue btw yet it is easy to try)
A key question when something goes wrong for me is "what changed between when things were working great and when they started not being so great." Quite often that narrows down the suspects of what is going wrong.
Also, a key question is if the problems you are having limited to just Photoshop or are you having problems with your system and with other programs. If its the later, then the focus should be on entire system problems and not Photoshop specific problems. The problems could be hardware or software.
Do note that I am not one to be on the "bleeding edge" of technology. Due to the endemic culture in software development around bugs (as in releasing software with plenty of them), I wait quite a while for the bugs to be wrung out before moving up to the most recent versions. I am still on PS 2020 and only recently moved to that from PS 2019.
It is also very important to make sure your graphics card is compatible. That means Adobe has tested the card with their software. If you graphics hardware and software do not support all that is in the following link, you are running without seatbelts: https://helpx.adobe.com/photoshop/kb/photoshop-cc-gpu-card-faq.html
If you graphics card is not fully compatible, you can totally turn off graphics acceleration in Photoshop Preferences for a while and see if that fixes the problem.
If you follow all of the steps in both of the links and still have problem that are only related to Photoshop (not your whole system and other programs), you can always post here for help yet it also may take some troubleshooting with an Adobe Tech.
Just some quick thoughts and hope they help
I just had a windows update come up and I installed it and it seems to have fixed my Photoshop problem as everything has relined itself and looks good. This is the second update in about a week so maybe that was the problem. Thanks to those of you that helped it was greatly appreciated. Many thanks Robert