Thanks Robin. Do you recall how many duckets a new core was? I agree it's prudent to stuff a new one in if you're in there. How much time should I allocate for the job? (And how many ibuprofin's for my back?)

Tiger - good thought. I haven't done the pressure test but I'm pretty much 100% sure it's the pipes/core. When under the car, you can see the antifreeze residuals by the header drains in the tranny hump. And all the other hoses in the engine bay are new within the last 2 years.