BMW: 1992 BMW 325I Convertible, radiator hoses, viscous coupling

Question
I have a 1992 BMW 325I convertible which I purchased last year with approximately 80,000 miles on it.  The coolant temperature has always ran between 1/2 and 3/4 on the temperature gauge when the A/C was on, but has started running that way without the A/C when driving at low (+/- 40 mph) speeds.  It actually overheated once when my wife was driving it.  When driven at higher speeds, the temperature stays in the normal range.  The coolant level is ok.  The auxiliary fan comes on when the A/C is turned on.  The primary fan appears to be working ok, but I am not sure how to tell if the viscous coupling is working the way it is designed.  The radiator does not appear to have any "cold" spots.  Any thoughts you may have regarding the most likely fix for this problem is appreciated.

Answer
David, The cooling systems on these cars are really tricky, not that they are bad, but they "CAN" be a pain in the neck to get right.
 As long as you haven't had the cooling system open, and by that I mean, radiator hoses off, waterpump change, coolant flush, or anything that might cause you to drain coolant out, then your fan clutch (or viscous coupling) is probably going bad.  The all do it, and it's like changing a timing belt, it's going to need it sooner or later.  
 These viscous clutches have a really thick oil, or silicone solution in them that changes viscosity with heat.  So when your engine is cold, just after start-up, the fan doesn't need to cool it. So the clutch is really "loose".  As the engine heats up, the fluid becomes more viscous and the fan turns faster, pulling air through the radiator.  Like Motor oil, this oil will eventually break down, causing the clutch to weaken, and loose efficency.  
 This sounds like it is your problem, and I would start by testing the fan clutch.  When the engine is cold, the fan should be easy to stop.  To test it, take a rolled up newspaper and slowly approach the spinning fan.  the fan should stop promptly with no damage to the paper.  I dont recommend this, but you should be able to stop it with your hand.  Drive the car til it's up to operating temperature, and then try the newspaper again (NOT YOUR HANDS) it should be considerably harder to stop, and possibly shred the paper.  If the fan stops as easily as when it was cold, your fan clutch is no good.  You should be able to find them at local parts stores, and I know they are all over the Internet.  Personally I buy parts from www.pelicanparts.com, they have good service and good prices.
 If this isn't your problem, let me know and we will find out what is!
 Take care, and good luck
 Josh Fields