As mentioned above, I'd definitely take Doug's advice first of all. Besides that, check that the thermo fans have been plugged in, and that they actually turn on when the coolant starts getting too hot.
Do those things first, and if your problem persists post about it again as that will eliminate the two most probable faults and then people won't have a reason to b!tch (otherwise, they will have every right to)

And take it to a workshop

Adi