You may need a more experienced hydronic heating contractor. I see no reason why this problem can't be solved. Perhaps the new boiler isn't plumbed right or there is an obstruction.

Replacing a cast iron radiator with a lower efficiency copper fin rad is a terrible idea. The copper finned rad will need water temperature 20F higher than the cast iron - and that is at a minimum!

If someone even suggested such a solution they don't know what they're doing. I know a larger copper fin rad can be installed to compensate for the lower water temperature but that would be a lousy idea. Fixing the primary problem properly is best.

IMO

SR

thanks for a quick answer. but how do i find an'experienced hydronic heating contractor'? web and yellow pages list heating contrs and i have had 3 of them here to no avail. if i ask if they are e.h.h.c. they would, of course, say yes. is there some way to sift thro and find experience and knowledge?

Did you bleed the air out of this baseboard radiator?

lily,
See if you can find out one or two equipment distributors in your area, and call them to see what contractors they recommend.