300tdi inconsistent overheating

Just recently my 300tdi has been getting rather hot on the temp gauge.. After replacing the thermostat i gave the landy a real good run working the engine fairly hard at a consistent speed of about 50-60 mph for around 30 mins with no problems at all (the temp gauge stayed mid way). Then tonight i covered about five miles and it started to get hot again and gradually reach the red on the guage. The engine has never had any overheating problems in the last year i have owned it. The oil cap and oil has absolutly no signs of emulsifieing etc and no signs of oil in the water. When the pressure cap is opened steam does seem to exit.

Has anyone got any ideas on what to do next or what the problem may be, as im unsure on the next step really. Any help would be greatly appreciated

check rad allover core when hot to see if cooling efficently and not down one side ,make sure it is completely full of coolant by removing bungs one on rad one on thermostat housing and fill and refit as each overflows excess will bleed out of cap harmlessly,check coonection and fit new sender on thermostat housing,