You could try disconnecting the top and bottom hoses of the radiator and flushing it through with a hosepipe, this may tell you if the radiator is ok at least. Afraid to say though, it does sound more like a water pump problem. I do remember someone else having problems with overheating which turned out to be the coolant pipe on the throttle body being blocked up, although this depends which model you have. AFAIK the TS type have the cable type throttle body which has coolant flowing through it, and the V6 has the drive-by-wire type throttle which doesn't.
Good luck getting it sorted, and as David says if you end up having the water pump changed get the GTV 3.0 V6 type as it is the only one with a metal impeller.
< Rob >
Spider 2.0 TS 1997
156 2.0 TS Selespeed 1999 (now sold)
147 2.0 TS Lusso 2002