Audi a4 sport 1.9tdi

[Eltel54]

I have audi a4 sport tdi 2002 I'm not a mechanic. I bought car 2 months ago guy told me there was a coolant leak cut a long story short there's to much pressure in cooling system  I've changed thermostat also put kseal in collant shortly after putting kseal in loads of foam in expansion tank. Could radiator be blocked car is now overheating but goes back to normal temp heater blowing hot and cold and losing coolant anyone have any ideas I'm puzzled