[1.8 CL '89] Heating Problems

Hi Everyone. I'm a new user, My name is Ben, and i'm from the Netherlands.
 
I'm the owner of '89 Golf 1.8CL. But i've got a strange problem regarding my interior heating.
 
When i start my car the heating will work for about 10 minutes and then slowly turn cold. And then sometimes during driving suddenly warm up again.
 
I Checked my coolant, and filled it up. But it doesn't seem to help.
 
Does anyone here have an idea or hint which might bring me in the right direction to fix this problem.
 
Thanks in advance,
Ben

How is it running in terms of engine temp. If its running hotter it would be your thermostat. other than that I'm not sure.

temps seem to be fine, never high