Hi,Welcome to Just Answer, My name isXXXXX will be helping you with your appliance issue today.
your magnetron is failing thats why its not heating correctly, and as far as the screen you would need to replace the
Control (smart) board assembly this alone is $167.00, and the magnetron runs about $130.00, the control may be the only issue but if its not then it will cost you more to repair than replace the microwave, these have come down a lot and you could get a new microwave for $200.00 to $300.00
here are the parts. magnetron.smart board.