I have a 2006 Ford E350 super duty. When driving, with the control on vent or ac, at about 1400 RPM the control switches to high heat defrost. When not accelerating, it will eventually switch back. Any ideas?

If your air dampers are controlled by vacuum I would suspect a vacuum leak.

that is Correct operation...it's an "Idle Up" that you need because the air conditioning compressor is taking away perhaps 18 percent of the horsepower for it's own greedy environmental concerns...think about a room air conditioner....the power....draws a lot of power...well the a/c is the same way....the motor needs more juice for the goose....like totally normal...I wouldn't complain...at least the air conditioner is working~

dandyoun is right....the vacuum operated controls have defaulted to a usable setting~ replace black butyl rubber with silicone for long life~

http://www.ford-trucks.com/forums/882747-a-c-moves-from-panel-to- defrost-and-back-under-acceleration.html --- check this out. It appears this is a common problem with vacuum leaks.

Here you go... https://www.youtube.com/watch?v=O-Wo9fgcBoQ

I have a 03 E-350 Super Duty XLT 15 Pass. van. After spending near 1K for parts changers, found the vacuum line THRU! the firewall was cracked, resulting in a vacuum loss to the climate control switch. Rerouted the line from the manifold to the supply line feeding the switch. Problem solved!! Now, problem with switching from vent to defrost, then back to vent, again low vacuum from the engine. Solve it by installing a vacuum reserve canister in the supply line under the dash on the passenger side above the area where the feet would be, preferably behind the air bag module out of the way. Small cost for the canister, less for hose, 2 hours labor to R&R.. Make sure canister has (2) two ports on it, one in, one out.