20.3 sound input problem

[w6jve]

I noted in the release notes that there may be sound problems in 20.3 and that it is suggested to use pavucontrol.
Well I've been using pavucontrol for a long time but this new problem has cropped up in 20.3

Machine is a Dell Optiplex 760 ultra small form factor. Running Mint 20.3 cinnamon Using the builtin sound device.
Kernel is 5.4.0-96

When I run pavucontrol it says the input device is microphone plugged in. Which is not true. I can click on the pulldown
and it shows line in unplugged. If I select that I get sound for a while, so it's not true that it's unplugged. But after a few
seconds to a few minutes the selection spontaneously changes back to microphone and I get no sound again.
I never saw this problem prior to 20.3