blowing hot when driving, cold at idle? anyone?

my 88 4runner (22re) blows hot air out the heater only when truck is at 2200 rpm and beyond. below that it blows cold air mostly, 92% of time. Replaced thermostat, not sure whats up, anyone help?

Sounds like there is a vacuum leak, typically in the diaphragm that controls your blender door. You'll probably have to trace the vacuum tubing that goes from the temperature and mode controls on your dash to the diaphragm's location. A hand-operated vacuum pump helps this diagnosis alot by selecting a nozzle that you can push into each hole in the connector and draw a vacuum. Good vacuum lines will maintain vacuum after stopping pumping.

Of course, if you have electrical mode control and a cable-operated temp control valve (located in the heater hose tubing near the firewall), I might suggest a partially blocked heater core or hose.