Originally Posted by bigblue2230
Try removing both the heater core lines and push compressed air through it. It might be a clogged heater core, so it only gets heat when the rpm's are up and it has more pressure to push through the clog.
It gets heat when it idles and loses it when you drive it. (heat at low rpm, none at high rpm).
If the cooling system didn't have a leak and wasn't low I'd bet 40 bazillion dollars it had a bad thermostat.