Exact version of PS? Exact brush settings? Simply sounds like you have enabled one of the jitter options in teh brush settings.
Thanks for the help. The exact version of PS is 2018. The exact brush setting is the spot healing brush, round, diameter: probably any, and tested with: 6, 8, 10, 15 px; hardness also probably any and tested with 20, 80. The behaviour was normal for several days after installing PS 2018, and changed only when I defined two presets, one for 15px 20% and one for 15px 80%. They both do the wiggly trick. From your help and some other help, I've found how to reset the jitter, and I can get straight lines again, but I can't find any explicit way to define any specific jitter in any planned way. I still have the presets I defined, so I can restore that jitter, but I have no idea why PS decided I wanted jitter with my presets. From other help, I've discovered that other people have had similar problems since June 2014. I can't find any panel or window or anything that lets me set the jitter for any parameter of the spot healing brush. I stumbled over a panel that might or might not let me do it for the healing brush, but it wasn't obvious how I might do that, and I don't want to do that, so I didn't explore that further. I tried HELP jitter, but like a lot of PS help, it was tormentingly unhelpful. If you know where the jitter panel is, please tell me.
Hello Mylenium, Again,
Based on your help and a lot of rummaging about, I conclude that jitter is documented and supported only for normal or nearly normal brushes, and that for the spot healing brush, it is an undocumented feature that happens only as an accidental side effect of defining a preset for the spot healing brush. Now that I know how to turn it off: Gear Icon in Preset window -> Reset Tool, it has ceased to bother me, and I can invoke it too, even if only in a clumsy way, and for scratches, jitter makes the artefacts slightly less conspicuous. I'd like to mark this as "Answered". Is that OK with you?