[fredjacksonsan]

Greg's got a great point....try the easy fix before starting a big job. Since the heat is only slow in coming (and doesn't blow only cold, as yet) the heater core is still flowing.

And while you're trying that, do your flush and fill for the cooling system if you haven't in a couple of years; the new coolant will help slow/stop any corrosion that's going on.

=edit=
Ups, just reread your original post, saw that new coolant added......