On the first microwave that flashes there should be a splatter shield on the wave guide inside the microwave. This guide cover has been contaminated with grease and food and it will need to be replaced. Remove the shield and clean the surfaces where the shield was and then test run the oven with out the shield and see if it runs okay
On the second one clean the roller rings and the path where they run both on the bottom of the platter and the microwave oven cavity. It is possible that the roller rings are worn out and will need to be replaced. The turntable is belt drive and the belts may be stretched out.