Overheating

Hello
Our 93 318IS overheats occasionaly when the air conditioning is turned on in hot weather. The strange thing is, it doesn't overheat all of the time, and it isn't dependent on the outside air temp!
The clutch fan has been replaced last year with a BMW unit and so has the electric fan. (I have seen the electric fan operating)
My question is, Does the electric fan have two speeds, and maybe the second speed isn't operating correctly or all of the time?
Thanks in advance for any tips.
Greg

I don't know about your model but the newer cars have a variable speed aux fan. you might want to check into it.
good luck
vern