When i turn my a/c on my 03 6.0 take offs are slow
The truck runs great other wise. Turn it off and it seems to quit the problem. I also noticed that if i switch it into 4 wheel drive the A/C blows through the defroster
Im not sure its all related but I figure I would put it out there.

 

Vacuum leak is causing the A/C to blow through defroster. Check to make sure it's all hooked up nice and tight on the pump on the passenger side wheel well under the hood. Turn the key on and turn the 4x4 switch and make sure you hear that pump kick on too. I've got the same issues with mine at the moment, though I don't have an issue with low power... May be related, but seems unlikely.