Well, that does kinda sound like a plugged heater core. The heater core does play a part in keeping the engine cold, so that could cause a slight rise in engine temperature. I don't think it could be a thermostat, since you would still get more heat as the car got hotter.
When'* the last time you had her flushed out and filled with fresh coolant? That may help, since it seems to be a problem that comes and goes (makes me think that it'* just crud foating around that sometimes clogs the core, sometimes doesn't). If you go that route, you might want to disconnect the hoses from the heater core and try to flush it out in the reverse direction of the usual flow, just to make sure it flows okay.
Welcome to the club, by the way.