Multipla 1.9JTD Over heating

My 1.9JTD Multipla has recently started to overheat or at least show it overheating on the gauge when i'm on the motorway particularly when the engine is under load. The fiat garage siad it was the waterpump so i had it replaced, however this does not seem to have fixed the problem. I'm not sure if this is related, however, when the engine is overheating the cabin air temp does not increase even when the temp dial is turned up to high - it is always cold. The AA ran a diagnostic and the coolant pressure is fine, the thermostat is cutting in when it should and the fan comes on. There are bubbles in the coolant but these were tested for hydrocarbons and the test returned negative, also no mayonaise around the oil filler - so i don't think its a gasket problem.

Has anybody had this problem before and know what it is and how to fix it.

I love my Multipla but i don't enjoy driving it at the moment as i'm always looking at the temp gauge.

Thanks