What you're describing happens fairly often with microwaves after a few years, so it's not unusual. The most common cause is either the turntable getting stuck or spinning very slowly, which leaves areas without constant rotation, or the magnetron (the part that generates heat) starting to fail and distributing energy unevenly. Before spending money on a new one, make sure the turntable spins smoothly without any friction, and try putting the food directly on the oven tray without the turntable for a bit to rule that out. If it's still the same or if the repair cost is getting close to what a new microwave would cost, it's probably more practical to replace it, but it's worth checking those simple details first.