the car only cools when we are going about 40=50 miles per hour
if both fans are working behind the radiator, why wont the car cool when idling on 2005 Mercury Montego
You might have a clogged radiator, or possibly a water pump impeller going bad. Check to be sure nothing is blocking the radiator inside the grill area. Also turn on your heater and see if you get hot air coming out if so then you you know you are getting some circulation.
I just had a new AC compressor installed and after a week the car started to run hot at idle. When I put the car in park and increased the RPMs to 2000 the heater indicator came back to normal. Come to find out when the AC compressor was installed they kinked or collapsed one of the coolant rubber hoses. Fortunately they fixed it and did not charge me. Lots of concern for couple of days.