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The fans in our cars are 2 speed fans.....slow and fast, both speeds controlled by the same relay, in the relay there is a resistor which controls the slow speed and is bypassed for full speed fans.....
Chances are the relay board has some bad solder joints or the resistor is burned out......
My 924s has the same problem but I haven't dived into it yet to fix it, if I'm stuck in traffic I just run my a/c which kicks the fans on high LOL
With the fans on slow speed it should be enough to keep the temps in check, when they're off there is simply no effective air going through the radiator unless you're going faster than 35mph......