my microwave went wacko

We were given this microwave two years ago by my parents. They had it a year before giving it to us-they were downsizing and had spent good money on a 'nice' unit.
Well, in September the display went cockeyed and pixilated. We could still read it and it was working fine. Two days ago, however, we put chicken nuggets into the unit for 1 min 45 seconds and left the room for the garage. The unit 'beeped' and I thought to leave them there for a few minutes and let them cool off for my son. About 15 minutes later we smelled smoke, and entered the room and found the living/dining/kitchen under a body length thick cloud of black smoke. The unit had NOT turned off, but the timer had stopped at 1 min 24 seconds, the unit continued to cook and would have if we hadn't smelled the smoke. The chicken nuggets were turned to coal, the plate they had been on broke in two and the unit was toasted. BE WARE! We still have smoke damage in the kitchen, the smell is horrid. It could have been a lot worse!