Front Window Defroster problems

My pride and joy is still a 1996 750il E38(I think) V12 with 80000kmMost of the time when I turn on the "Front Window Defroster" I can never predict whether it will blow full force HOT or COOL (it does what IT wants). When it blows HOT I can only keep it on for 15-30 seconds (it becomes too hot inside the car)Can anyone suggest what the problem could be?Thanks in advance