Could be the fan thermoswitch. The thermoswitch, if there is one*, is located on the lower portion of
the radiator just in front of the turbo outlet pipe. Here is a
picture.
You may also have one of the fan relays failing. (On some cars, there were two relays, one for low speed and one for high). Here is a
picture.
For general diagnostics of the cooling system, go
here.
2 stage cooling fan wiring diagram
here. This
also will tell you where to find other components to check, like the
fan speed resistor, and provide other diagnostic checks.