heat problems

I recently brought my lr discovery II in because my heat was not working during cold weather. I was told by the tech that my fluids were low and that was why I didnt have heat. The next day while driving I had heat for about 10 minutes then it started blowing out cold air. Seems as though on warm days I have heat and on cold days I dont wondering if this is a common problem?

Check the coolant again.....

Coolant seems ok. I have recently notice that if I am accelerating or going up a steep hill my heat is on full blast but if I am stopped it blows cold air out of my vents.

Check for a vaccuum leak on the heater/ AC vent controls.