Coolant fan problem

I have a 96 quad 4 Grand Am. When I am moving, the engine needle goes up like it is going to red line. I have noticed it go up and come down several times. I usually don't let it get too high. It never used to do this. I bought a coolant sensor and installed and it still functions the same way. The problem appears to that the coolant fan is not coming on the way it should. The funny thing is that it seems like everytime I take a meter out and read the ohms at the sensor I see a change, and then when I look for voltage at the connecter to fan I see about 13.5 V. Plug it in. The fan comes on. This bugs me because everything seems to be working however not the way I need it too. Can anyone suggest why it isn't coming on and off the way it should?

It could be a bad temperature switch on the fan itself. Go to the local salvage yard and by an new fan assembly from a newer model car. This should be relatively easy to install and will probably solve your problem. There is one other question, have you replaced the thermostat? The one in the car could be sticking and that will also cause the temperature of the coolant to rise and fall as you described. Both conditions can cause engine problems if not corrected. I would start with a new thermostat and if that doesn't work then get the new fan.