Hi Staples,
I am having exactly the same problem, been searching for months & months and I have read every post I can find, I have flushed matrix, fitted new thermostat, checked rods, and all is fine, a few weeks a go I had a nice 48 degrees with a temperature sensor in the vent, now I am lucky if I get 4 degrees, I am having another go tomorrow and will definately let you know if I find anything. Mine seems to work for a bit if I force water back into the matrix outlet pipe whilst running and then quickly put the pipe back on, but after a week of driving I loose all heat.
Steve