I have a 2013 GT Prem that did the same thing. Build date 1/21/2013.
My clutch pedal assembly had 2 springs a return spring on the brake and the helper spring on the clutch. I did not have a return spring on the clutch.
This is IMO the most annoying thing about this car.
I removed the helper spring, better but still stuck to the floor with spirited driving.
I installed the high flow high pressure clutch line. Better but still did it.
I wanted the return spring I bought a new pedal assy to get this.
DR3Z2455A is the GT500 pedal assy. Perfect fit. It had the helper and return spring a 3 spring assy.
Installed, bleed the system. I used a vacuum to help bleed the system.
My clutch problem is gone and the clutch feels and acts perfect now. Three hard pulls through the gears and no issue at all.
Find a way to bleed the clutch system with a vacuum before you do anything else. It says its self bleeding but it's not completely. After bleeding the clutch was much more stable. It would get soft after it sat undriven for a day. It's firm every time now. The return spring was not that strong but it does help.
Based on what I've done the problem occurs because the return pressure on the clutch drops below the pressure of the helper spring leaving the helper spring dominate until the rotating mass slows and the pressure returns.
Ford knows there is an issue, there have been over a dozen pedal part number changes in 3 years.
I don't know the part number for the 2 return spring non helper spring version. I got the 3 spring version and took off the helper spring. I have two helper springs now if anyone wants one! Lol Sent from AutoGuide.com Free App
[B]2013 Mustang GT Premium 5.0 [COLOR="DimGray"]Sterling Grey[/COLOR][/B] 6MT 3.31
35% Tint, MGW Shifter, JHR Hectic Motorsports High Flow Clutch Line, Borla S-Type Cat-back, Airaid Race CAI, Bama 93H Tune, Roush Chin Splitter, GT500 Style Spoiler, GT500 Pedal Assy, Ford Racing M-FR3A-MGTAA Suspension, 5W-20 Royal Purple API, High Performance Key Fob Battery.