Rahray

The heater on my 93 400E has been acting strangely lately. When I turn it on it acts normally blowing hot air out the two side vents. After 10 to 15 minutes cold air will begin blowing out the center vent. It will eventually stop but starts again imtermittently turning off and on. I'm familiar on a basic level with the climate control and wonder if this is more likely caused by a failed sensor or a failed mono valve? How might I go about diagnosing which it is?