darktable 3.8 flakey, where can I find the last 3.6 for windows?

I think uninstalling and reinstalling makes little sense, unless you think the installed software got corrupted – but why would that happen?

Starting with a clean setup (either by moving the existing config + DB files, or pointing darktable to an alternative, debug location using command-line arguments) could help.
Running from the command-line has other benefits, e.g., one can see logged error messages there.

Disabling OpenCL would also be one thing to try.

To learn about command-line options, read darktable 3.8 user manual - darktable.