My 155 (2.5 TD) also runs VERY hot.
Driving faster than 100mph will cause the engine to overheat (+100° water, 120° oil), also the warning light for overheating occasionally kicks in...
Asked an Alfa dealer here in Germany, they said its a problem of the V6 and the 2.5TD, they were not built for the german freeway (!)...but when the outside temperatures drop, I could drive fast again - now that's what I call technical assistance.
One of the mechanics races a 155, he conspiratively told me to chuck out the OEM rads and substitute it by alloy rads, that can be found in VWs...
Next spring I am going to give it a try - if the car isnt sold by that time.
I can promise you nothing but blood, sweat, oil and tears
75 TS 2.0