need some Heater help

My 2000 voyager once it is up to normal tempature I can not get any heat from the core. But when I excellerate or drive normally the heat will come one. Can anyone tell me what is going on? I thought that it might be a air bubble in the core or in the rad. Please help....

Probably the thermostat not opening soon enough? When you drive it or accelerate it warms it enough to open the thermostat. Another thing might be that the core is plugged. You could have it sucked out. Mine had a bunch of jelly like stuff in it once. My bet is it's the thermostat though. Not much to change it.

Click the pics to blow them up.
http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c152800420fe