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So it sounds like your cavity thermal fuse has gone bad which will be located up above the microwave cavity and likely the $5 part you replaced last time. The part number for the thermostat that is going to be bad is part number 6930W1A003J. Below is a link to the part.
CLICK HERE for thermostat
Now there are 2 reasons this thermostat would go bad when you're using timer mode. One possibility is that you're accidentally setting the timer incorrectly and the microwave is coming on when you tell it to heat. The other possibility is that you just have a bad timer which is turning the microwave on when it's not supposed to and the logic is bad so the entire display board would need to be replaced to resolve that issue. It's not common for the display board to fail in this way, but it commonly happens then when someone uses the timer they hit the wrong button and the microwave comes on instead of just counting down like normal so if you think that you may have done this, just replace the thermostat and be careful in the future to not do this again. If you don't believe there is any way that you could have set the microwave to heat rather than set the timer accidentally, then replace both the thermostat and the display control board.
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