Ask a garage how the pump could be checked, I would say it is unusual to overheat so quickly if the pump is OK.
If that's OK then suspect the thermostat (but you changed!!)
So then .......the sealer (Radweld or similar)
Radweld is a radiator sealer - really just intended to be temporary.
Try running engine until 'overheating' and feel your radiator. Is it hot across the full width? If so it's doing it's job (does the fan cut in?).
If not it is probably (or partly) clogged with the sealer and the water is not going thro' ALL of it therefore giving the rad chance to LOSE the heat it doesn't need.
I also asked does the heater work well, if that is poor then the mattrix MAY be partly clogged as well.
If the rad weld is at fault then any flush may not fully remove the clogs (including the leak area), but if they ARE removed then you MAY know for definate if that WAS causing the overheating, even if you get your old leak back!! So then replace radiator.
Steve
March 2007