Pretty rare but I've seen hoses block flow as well when the inside shreds a little and forms a flap that blocks flow. You could gain information by checking the LP pressure at the intake of the regulator when it's running poorly. It's the 30 seconds that makes me harp on the LP supply.

You do have full coolant level I assume, and nothing is frosting up.

Also, is there an LP solenoid? You might want to make sure it has power in run as well as in start mode, though I'd expect that to cut- out running altogether.