Anders,
You mentioned (if I remember correctly) that his just started happening recently and was O K before that. I think we may have over thought this one. One question, then a possible solution. Is the fan controlled only by the thermostat, or is there a fan switch that will turn the fan on manually?? If you have a fan switch, turn your key to the on position without starting the engine. Flip the switch on and check to see that the fans are really turning.
I noticed yesterday that my temps, while moving in stop and go traffic, were creeping up to 180-200F even with the fans turned on(I have a manual switch). It usually will climb no higher than about 170-180. Checked my water level this morning and it was low. I don't have the negative pump for filling so propped the car on its butt and had the nose in the air above the engine level. opened the vent line in the radiator and added water to that port(my radiator has a plugged vent line on top and a return vent line on the rear face,outflow side of the radiator). Closed it off and ran the electric water pump. for a few minutes, then rechecked the vent line and had to add a little more water. Sealed it all up and took it out for a drive. The temps remained normal for the drive. Parked it in the driveway and left it running with the fans going and ran into the house for about 5 minutes, maybe 10 at most. When I came back out the temps were close to 200. Drove it around the block and the temps settled down. After parking the car with the engine off I did the above procedure, and sure enough the fans were not coming on when the switch was turned on. Dropped the dash and found the push on wire connector had become dislodged. Hooked it up and all is well. It may be something as simple as that. My engine is somewhat noisy and I can't hear the fans or the water pump while moving.
If you are controlled with the thermostat, crank the engne. When it passes the thermostats trigger temp, check to see if the fans are moving. If not, then its the fuse, the relay or a wire to one of them is off/broken/loose, or the thermostat is defective. "Occam's razor"
Bill