but have you felt it when hot to make sure its hot allover core,its usually poor rad or lack of checking real coolant level using the 2 bungs that boils 300tdi not prone for it apart from them 2 reasons
only in that 200 rad fills and empties on opposite sides 300 on same side,apart from that they are equally good engines,check before going too far in new one ,otherwise it will be 4 ,200 will bolt in okay apart from welding new mounts ,new exhaust system 200 rad and pipe work for cooling system and intercooler
Have you checked the rad properly? It's not unknown for people to clean mud off the front but the back of the fins are still clogged solid. Take the rad out and cowling off, flush the inside of the rad both ways with a hose then make sure you can see daylight through all of the fins.