1991 dodge van heater blows hot and cold

I have a 1991 dodge 250 and when it warms up as long as the idle is up it blows hot air but as soon as the rpm's go down it blows cold air. Also there is a device that when you go up hill it stops blowing on ac and heat i was told its to send power to the engine is there a way to stop this from happening

I have a 1991 dodge 250 and when it warms up as long as the idle is up it blows hot air but as soon as the rpm's go down it blows cold air. Also there is a device that when you go up hill it stops blowing on ac and heat i was told its to send power to the engine is there a way to stop this from happening

Sure you don't have a vacuum problem to the dash control?
http://dodgeram.info/tsb/1992/24-06-92.htm

When's the last time the thermostat was changed? That could be a sign of waterpump failure, too. When the impeller blades get worn, they don't pump very well..

I changed the water pump less than 6 months ago and the thermostat is working I did find a leak last night but when the radiator is full it still blows hot at high RPM's and cold at low RPM's

You may have a heater core on the verge of clogging up. Does your heat gauge move any when this happens?

The temp. gauge stays the same is it possible that the leak releases enough pressure that it stops water flow

A leak bad enough to lose heat would be a gusher. Although, if the cooling system lost pressure, the coolant would boil easier, allowing air into the system.