It's only reporting L2 voltage, up at the control. SO 120 is ok I think.

From the beginning:
1) Are there any burns or other odd marks on the plug it self where it goes into wall? Each post that goes into the plug should be clean and have no scorched or other spots. If so you have an issue inside your wall plug.
2) What else hasn't been working right? Poor heat?
3) If you have a multi-meter, is the problem often enough that you could test the voltage at the board and see if it is indeed dropping off? You'g monitor it at the live side of the heater relay I think.
4) look at the connections at the terminal block where cord attaches to the dryer, a loose or failing connection there would cause this. It's common enough that I take a quick look when I see this error.

However, I'd guess your main control main relay is failing. The service manual says replace the user interface first but I doubt that. I've changed many main controls and few UIs.

Your manual: https://1drv.ms/b/s!AsrelWF4HEkViRdDpsvHjXqzoYpl

Likely part: Whirlpool WPW10111621 Electronic Control Board - AppliancePartsPros.com

It's a pricey part so I'd be fairly certain that it is the issue before I spent for it. Hopefully you can catch it in action and verify that you are not loosing one side of the 240v.

Are any other 240v appliances working normally? I had a bunch of 240v related issues in my house a few years ago, range, dryer, water heater...it drove me mad trying to solve. Finally I realized the the only common link was the service, the utility found a damaged connection on the roof and solved it.