WTF is this sound, pinging?!

As far as I know the Mustang fuel system and computer is not capable of detecting the level of octane and then adjusting the timing and whatever else to prevent the ping, and increase or decrease power. In fact I am almost positive of this fact because my family has had cars which do this. I really wish the mustang had this capability, because my car does ping on occassion (and this was prior to my mods). Prior to all my mods the one or two times I did put in premium gas I too felt that power was cut. Anyways, since you've tried the high octane thing, get it back to the dealership, the few mods you've done should not void your warranty in any way. The one good thing is that it sounds like the problem is very consistent, which means they should have no problem finding the issue and fixing it.