you're gonna have to do a comp test mate..

If its running on 3 tho it could be the lifters if one of em is out it might be loosing comp.

That Xr3i that Kipper had ( I think u had it before) was like that when I went to look at it, creamy froth pissing out of the header tank.

Said it was fine after he'd cleaned it out, try flushing it out, put some new anti freeze in it and run it for a bit see what happens.

Jake