GE Advantium F-3 message-nonfunctioning microwave

I have an Advantium Oven model SCA2000BBB 03. It was manufactured in 1999.
Early on, the microwave componenent stopped working (after perhaps a year). We’ve been using it as a conventional oven.
About a year ago the oven began shutting down during the cooking process. It would beep and display an F-3 message on the display. I’d unplug it for a few minutes and it would work again.
Now however it shows the F-3 message doesn’t operate normally after unpluging the power cord. The clock feature will blink. Rotating the main control button will allow me to change the hour but it doesn’t respond when I push that button to set the time.
Is there anything that I can do to get the appliance working again? I shudder to think how much in home service will cost on this thing. Purchasing a new one is not possible at this time.
I expected a lot more from this in terms of quality and performance. The plastics are cheap and prone to cracking. It was way overpriced and still is.
Thanks in advance
Frank