ford galaxy temp. ?

hi, i have a n reg galaxy, when i start car engine slowly warms up and temp gauge moves to mid-way, and i have warm air. but as soon as i start moving the temp gauge falls below zero and i have cold air, if i stop for a few mins it goes back up, but move again an it drops,i have replaced the thermostat on top of engine but makes no difference, would appriciate any help.local garage said it was engine management system?. thanks in advance.