Weird cooling system behaviour

My 318 takes too long to heat up, in fact just gets over the first line around 120F, I've only seen it get up to operating temp once. It should get there quickly, I'm mostly driving around town.

However if it gets in between where it 'should' be it starts to steam out of the overflow hose off the rad (I don't have a reservoir)

I have a manually operated electrical fan but if I leave it on all the time temp NEVER gets above 120F

Cold motor is no good. Methinks open thermostat, but what gives with overheating behaviour at normal operating temp?

Also heater vents give warm air on the ride too, and cold air on the ride back or vice versa, it's random.

I've only had the car a few weeks and have put about 6 quarts of coolant in myself, alot of which leaked out or steemed of it seems, always comes out brown and dirty either way.

Clogged system? Too much water in the system perhaps from PO? Thermostat definately stuck open right?

Advise coolant flush and thermostat replacement? Am I on the right track or overlooking something?

If so how quickly should a 318 get up to operating temp (outside temp 40F) at idle with a 180F thermostat?

How many quarts to flush the system and what's the best way to flush the whole thing properly including heater core?

David