mk2 ford mondeo heater tripping out?

hi a few weeks ago the dial that controls the direction the air comes out of the different vents stopped working . No matter what position I put the control dial to the air would only come out of the windscreen vents.Now when I put the air speed onto 3 or 4 it runs for about 30 seconds then seems to trip out?As the cold frosty nights approach I need to try and solve the problem any help would be greatly appreciated.If I wait about a minute it works again but then stops as before thanks