i own a whirlpool over the oven micro/hood,sometimes it heats things and sometimes it does not, everthing functions like a normal cook cycle,but it seems random when it will heat things. it may cook a bag of popcorn one day,then 2 minutes later it wont even pop a kernel.any ideas what this could be and if it is a repair that can be made at home?this micro is only 2 yrs old.thanks