Re: That sounds more like thermostat stuck open,
+1 on the T-stat. Same thing with my 528 when I got it. I agree about replacing the housing and the water pump as well. Just make sure you get a decent pump...one with a metal impeller. I forget what I got but I think it was German. The cost of the housing is minimal and worth replacing while you're doing the job IMO.
IMO, you really shouldn't drive around too much like that because your computer will run the motor rich which will not only give you poor fuel economy but also eventually lead to other problems like your cats burning out, and possible cylinder wall damage from running rich all the time.