Still not getting warm

Yup, I thought I had it fixed but apparantly not. The car in question is an 02 TDI Disco and the problem is that there's cold air (and sometimes very hot) pumping out of the interior heating. The coolant was slightly low so I topped it up, but a few weeks later it was back to it's old self. Driving along there's no problem but sometimes the temperature gauge suddenly drops to the blue level and the warning lamp comes on. I can't see any residue from the pipes either so could it be the thermostat.

Cheers
Al

Warning lamp? Which one?

Not exactly a warning light, the little red light above the temperature gauge. I filled it up again today with normal water (needed about two pints) and it worked fine. Question is, I only filled it a few months ago so where did all the water go...could it be the overflow?

Al