double whammy!

I'm sorry, but these vehicles are keeping me awake! I have a 97 F-250, 5.0, auto. 4x4; and a 2000 E-250, 4.2 auto. 2wd., and they have the same problem!! The air is switching from vent to defrost to floor on acceleration!! I have checked all vacuum hoses, and they are fine! I thought, maybe the control switch; but it is only electrical! so, there must be a vacuum control module somewhere under the dash? Please Help!! Thanks to all!!!

I'm not positive about this, I've never seen one fail, honestly, but I think the feed vacuum one-way valve isn't working or is missing

I'm not positive about this, I've never seen one fail, honestly, but I think the feed vacuum one-way valve isn't working or is missing
In addition to this, there may be a small vacuum leak upstream of the one-way valve.
My '87 Mustang has such a leak and does exactly the same thing as your trucks.

Thank's for the input! I have since place a vacuum gauge in line to the black hose at the switch, and drove both vehicles to see what happens with vacuum on acceleration. I'm from the old school so I know with our old vacuum wipers they would stop dead on acceleration! (Ah! the good old days!), but back to this problem; with the gauge installed and accelerating; there was only 5" vacuum loss, from 15" to 10", natural occurance! So I'm now leaning to the control switches themselves! I will replace these today, and will post the outcome! Thanks!, and Goodnight!