I just fixed this problem last week on my '96 LS400. The cold air blowing at low RPM's is caused by air in the line that gets trapped at the high point, which is where the heater control valve is located. When your LS400 is cold, start it and run it for 5-7 minutes at 2500 RPM's... adding coolant as necessary. You should see some air bubbles escaping from the coolant tank. Once you run for 5-7 minutes, shut off vehicle and continue to add coolant to system as it cools (the coolant will be sucked into the engine coolant system for several minutes). You may need to repeat this process once or twice (1 or 2 days apart) to ensure you get all the air out.
