Volvo Repair: Temperature, volvo s40, water pump

Question
I have a volvo s40 2000, i don't know why the temperature goes up and
down. After been driving for 25 or 30 minutes, the temperature start to goes
up and then the second fan start to works and lower the temperature to the
middle. A friend told me, it could be the water pump, maybe the the radiator
or a bubble of air. Any help i will really appreciate it. Just in case I live in Las
Vegas, i used to live in philadelphia.

Answer
Weinert, Tell your friend to read more books on auto repair.  This sounds like a ECT sensor issue.  I live in Florida and those sensors are always going bad on those cars.  You may also have a sticky thermostat. That should be done when the sensor is changed anyway.  Roger