Transit Courier 1.5 heating problem

[Appie]

Hello.
Please help with cooling problem.
Few times a month it seems that my engine is very hot. Cooling fan keeps running when I take out the car. This already happens after a short ride of about 8 kilometers at 6 degrees outside temperature. So the engine really shouldn't be that hot. It also doesn't happen very often just once. Anyone an idea what this could be? Does the thermostat stay closed? A normal temperature is simply displayed digitally on the dashboard. When I'm outside and the fan is running, you really smell a warm plastic air. So it is very hot there.

Thanks.