The ECU controls the fans based on a/c gas pressure (a/c on) or based on coolant temp (a/c off).
I suspect your slow speed resistors (the blocks mounted top and bottom on the rad fan mounting frame between the two fans) are faulty, a common problem;
If temp gets high enough (around next mark past 90 = 110) the High speed fans will kick in (a/c off) but with a/c on it's demanding hi speed anyway - unsure if this is normal or as pressure is hi enough (due to no slow speed) it is demanding full speed anyway.
Other cause I can think of is the temp sender to the ECU is separate to the temp sender for the gauge, as temp on gauge looks about right for a/c on, could be the ECU temp sender has drifted?
I'd try a short piece of wire with spade connectors each end to jump the two terminals of the plug going into the low speed resistor - this will make fan run at full speed when there is demand for low speed, rather than not run at all (i.e. if resistor is open circuit). I don't follow Binzzy's idea that it caused his temp sender to blow - totally unrelated electrically, it's just the same as turning fans on full speed without slow speed working anyway, the only issue this might cause is increased wear on the slow speed control relay - but it's the same rating as the full speed relay anyway. It's a common test / temp fix from what I've seen on the forum and I had in "temporarily" installed for about 6 months with no problems!
See my previous post on a similar problem - http://www.alfaowner.com/Forum/alfa-...mp-sender.html
Only worrying thing is, my temp hs now started to run between ~92 and 100 again when stuck in traffic. Will have a proper test whenI get a chance to see if other resistor has gone or if I can find any other issues. I hav alfa diag so will try to plug that in and test and read ECU temp sender reading to compare to dial.