Q: Van is overheating and not blowing hot air.

My van all of a sudden started overheating. I replaced the thermostat and radiator. I also checked all hoses, water pump, and made sure the fan was kicking on properly. All of those compnents work. The oil isn't milky so the head isn't cracked. Went to flush out the heater core and brown stuff came out that smelled burned. Thought that might have solved it, but still having the same issue. The funny thing is that when it idles, it's fine, but when the rpms go up, so does the temp. As soon as I stop, it takes less than a minute until the temp falls back to normal range.

My car has 133000 miles.
My car has an automatic transmission.