Over heating?

Hi, I have a daf cf 85 tipper truck I had to change the termostat due to it being faulty not working. Since fitting the new one the truck has ran hoter than it did before.before changing it the temp gauge usally ran just over quarter and when engine came under a pull it would go up to half. At moment in runs at half which seems pretty normal and when comes under pressure it rises to 3quaters on the gauge? It never goes any further and always returned to half fairly quick.the problem now is that it keep throwing out a little water from the header tank through the over flow pipe on header tank it can be visable on the grill of the front off truck. It only goes to a certain level and then not near as much and yes I don't have the header tank to full it's filled to correct level. It always sets of the coolant warning light in dash. Prob uses between half litre to a litre some days depending on the job. I have changed radioator cap and changed the radioater just to rule out these. My mechanic wants to do the head gasket but I'm very slow to go down that route as this prob started 7 months ago and hasn't got worse so I think that's not the problem. Maybe I'm way out but would it be possible the new thermostat is letting the engine run a lot hotter than the old one hence the problem??? Would be for greatful for any advice or solutations. Cheers lads