if it was me i would smell it to see if it has a burnt smell,then i would open to see if i could see any spots on it that looks burnt. i would also run my hand down the cord while its been on for awhile just to see if its hot.when you open it make sure to blow out any dust,try running with out it being in the case to see if it does it again if it does i would send it back or replace it if it does not happen again i would do what they said in the above posts.....because you said it has not happen in the last year and that you have had it,sitting in the same spot i doubt its a cooling problem unless the cooling vents are plugged with dust...until you do figure it out i would unplug it when not in use if its getting real hot plastic burns just to be on the safe side
i was typing when you posted i hope that solves it