as merely another option is hasn't been unheard of for the cooling fan(s) to quit their normal mode of operation.. either through a faulty plug on the bottom, or a faulty ciruit, etc. It could be a fan not turning on on the low end, and then once the car overheats enough, the HIGH fan turns on, cools it to just below its (high fan) turn on point, and then shuts off.. and up goes the temp, again..
As your car is idling, check for these things:
heat from the vents.. make sure it isn't the water pump that is out (although you have the zetec, which isn't as infamous about the wp)
pop the hood and see what's corresponding with the rise/fall of temp... are the fand kicking in? mine are LOUD if you are idling in a lot, somewhere... you can hear it for sure.... if they are working, and you have heat... look to the thermostat, next... not too hard to replace, and inexpensive as well..
Ray
