2002 BMW 318 air condition

2002 BMW 318 4 cyl

hi dear
the proplem is that the air condition fan is some time stopped suddenly and it doesnt work.some time i turned off the condition ane then turn it on ,the fan work again .but some time it does not work and it is still long time not work.i took my car to maintenance ,they change something and its work again but now the proplem return again
can you help me