I have a 4.0L v6 2007 mustang. It shifts into overdrive just fine but at WOT It goes 1,2,3 and when it hits 4th (overdrive) It flys up in RPM and doesn't put any power down so I immediately stop accelerating to prevent damage. Its almost as if the clutch just disengages.
I have a diablosport 93 octane tune, bbk cai, pypes performance exhaust and xpype. Could anyone help me figure out what exactly is up with my car?

 

an 06 auto has five gears. 1,2, 3, D (4th), then Overdrive (5th) that is controlled by the OD button on the right of the shifter. You can hit the button to turn OD off/on with the pinky finger of your right hand.

When did the problem start? Have you checked your fluid level? These cars do not have a dip stick so you must get all four wheels up in the air and get under the transmission to remove the center hex screw in the drain pan to check the fluid level

Are there any lights on the dash when this happens? You may want to get a scan with a handheld that can read OBDII and transmission codes to see if there are any Transmission codes that are pending or have been stored.

If it's not the fluid, it could be either valve body or solenoid related. The solenoids are the electronic brains of the transmission and control the 'gates' in the valve body. The valve body contains the gates that the solenoid pack controls to control the fluid paths through the transmission.

 

When I had a shifting issue w my 06, 4.0L, I had the transmission fluid change by a local shop and that cleared up my issue some time ago. Still shifts positive today.
What did you end up having to do so others might learn?