Replace the heater control valve, it's leaking so there's not much to do there; now, did you checked all the vacuum lines in the engine bay? all those lines are connected to the same vacuum source that the air conditioner control is connected to, which means that any vacuum leak will have an effect on the system: EGR, Fuel Recirculation, Air Conditioner, Heater, Brakes and PCV. If you checked and found nothing, check again and replace the heater valve; check the lines that go into the passenger side bay, you'll find two lines that come from the firewall, a white one and a black one and a connector; it's very easy to hit it with a foot and disconnect it, so check there before going to the shop; your pocket will be thankfull!
Those white and black lines go to the control know in my X-sport, I have manual controls, so I'm not sure if it's the same on yours which is automatic.
The other thing I would check is all the vacuum lines in the dash; there are a few valves in there, any of them could be disconnected, broken or blocked, just check to be sure.
Like I said before, I'm completely sure that in my X-sport with manual controls for the air conditioner the problem was in the vacuum lines, I fixed the lines and I fixed the problem. Every time I accelerate it would start blowing air through the defogger and the front ducts would close, I checked all the lines and after fixing the lines and placing them in the right way the problem stopped, the CEL light was resetted with a battery pull and have not had that problem ever since.
Today it´s a good day to check those lines; inside and out; any vacuum leak will cause that problem when you accelerate; the engine creates most vacuum when at idle, and the least vacuum when accelerated.
I hope my experience can help you with your X.
Later we can talk about that heater valve, in Venezuela we have no cold weather, at least not where I live, and it's difficult to find a heater control valve because it's a no sell... so I just removed it, put a couple plugs in the water pump and the heater core, and now it's good to go... Just in case you don't need that valve or found no replacement.
Any questions I'll be arround.
Eiber.
(yes I put oil in the unit and it did last 3 years) This time will put a brand new one on and once again go over as much of the vacuum lines as possible. 
