I second Martin's old school base line check.
In fact at this point I'd pull the thermostat just so I knew it'not involved at all. I'd fill the system right there at the thermostat/hose... garden hose into rad return hose till it flowed out from the block with no air pockets, hook it up and and do his check.
Overkill? Maybe. But this thread is starting to eat it's tale and his test lets it die and be reborn as a question about it's answer...
