Welcome to the forum.

Sounds like a thermostat problem.

You did not mentioned if you got that changed at the same time as the other stuff.  Pity cos you need to take the fan clutch off to make room to get to the thermostat housing.

If your temp gauge goes up and down at the same time as the heater blows hot and cold, this will confirm a thermostat problem.  I had this on my E39.  New stat cured that problem.

The engine will get hotter under load (acceleration) and cool down slightly when you are off the power or coasting.  My old Vauxhall Senator used to do this.  The heater was hot on the Motorway and I could feel it getting cold when coasting on the slip road leaving the Motorway.  Problem went when I changed the thermostat.

Remember to get new seals and gaskets fitted when changing the thermostat.

Hope this helps.

Andrew

What was the original problem? Same as the current problem?

BTW; Hello and welcome

Hmm so you got a thermostat fitted then.

Could be something more serious and sinister like a cracked head or cylinder head gasket gone. 

I would get a compression test done on your cooling system, that will check for any leaks.

If you remove the oil filler cap, what does the underside look like?  It should look brown/black oily and not white like mayonnaise.

Andrew