Heater cooling sys mystery

[DanaT]

2004 s60 turbo. Has leaked coolant since I got it 3 years ago and getting worse. I add about a qt per week now. No visible leaks. Had mechanic put dye into system and cannot find any leaks. He suspects system not circulating properly, building pressure, and pushing out over flow (more on this below).
Heater stopped blowing heat (cold only). Had core flushed because one hose hot the other cold. Had thermostat replaced. Now heater works very little at idle but ok at 2500 rpm. Both heater core lines indicate flow thru heater core. Both radiator hoses cold, even when temp gauge reads normal.
Now suspect water pump or blocked radiator? Interested in your thoughts.