[replicant_argent]

Stat, control/blend problem (maybe), partially plugged core, water pump vane erosion, a few possibilities.
when it sits at idle, can you get to the return line from the heater core? if it is warm, you can isolate it to behind the firewall, whether a heater core, or a HVAC problem, rather than pump/radiator/stat/etc.

I would toss a stat in first, as it is cheap, easy, and if you haven't done one in a couple years, might as well. Especially since you had a cooling problem recently, and it may have boogered the stat.