Hi @bobcohen
I suspect that the electrical contact surface on the underside of the Start/Stop button where it comes into contact with the contact membrane strip has been worn away with use.
Unless you can find a replacement keypad for the oven (I can't find any parts at all for your model) you could try disassembling the oven to get to the keypad, disassemble it and then try coating the button's contact surface with a [https://www.google.com/search?client=firefox-b-d&q=metallic+paint+for+electrical+circuits|conductive paint] to see if you can prolong the life of the oven. (examples only)
''***Be safety aware*** when working in microwave ovens. The ***HV capacitor in the oven can store >6000V DC*** for months even if the power has been disconnected for this length of time. ***This amount of voltage can seriously injure you.*** The HV capacitor needs to be discharged correctly, as soon as it can be accessed after the oven's cover has been removed and before any further work is carried out inside the oven''.