Lexus GS 2006-11 :: Heater Blowing Cold At Idle, Warm When Revving / Driving

My '06 GS450h has developed a fault where the heater blows cold at idle and low revs but blows warm when driving or revving the engine at a standstill. The coolant hasn't dropped at all and the car never goes above halfway on the temp gauge. Its starting to get frustrating now the cold weather has set in and temps dropping! Long journeys are ok as at high speeds it stays warm but short journeys are uncomfortable!

I just had a feel of the rad hoses and hoses that go into the matrix (engine side of bulk head) and matrix hoses feel warm (both of them), the top hose to the rad is warm but not hot and bottom hose is cold suggesting the stat hasn't opened yet. Its cold outside so getting the car up to temp is quite difficult without a long drive (rush hour at the mo).

Still I just had the car parked on the drive and cranked the heater up to max and held the accelerator for a while and super warm air blows out. The engine cuts back out as "normal operating temp" has been reached for battery mode and then the heater goes cool again! Still the bottom radiator hose is cool and top hose not massively warm. It has never overheated thus far and I drive 150miles a day back and forth to Reading from Cheltenham 4 nights a week.

Only thing I can think of is a sticky thermostat or water pump not operating sufficiently enough to circulate the water? Its getting colder and short trips where it used to blow warm air nice and quickly (within 3-5miles) are becoming unpleasant!