1996 Dodge Caravan my heater blows heat weakly now and so

My minivan doesn't blow heat very strongly today. In the morning the check engine light came on. I opened the hood, checked the resovour and added coolant. By the way the car never overheated. A few minutes later, I turned off the car, and 20 minutes later turned it back on. The check engine light went off, and the van blew very weak heat.

there were times that the heat came out strongly. The fan is running like a champ, the van is maintaining temp, the heat comes on and off once and a while.

do I need a new themosdat, or radiiator cap? If its something else, I will need a diagram to fix it. Thanks