4th gear issue

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On the 1/4 mile, when it's time to shift from 3-4, once I go into 4th, there this slight hesitation, like a slight stop as if the power isn't going to the transmission. This only happens for a fraction of a second.

I would think it's the stock clutch slipping, but I did try the test to determine if it was slipping and it doesn't.

Any idea what this could be?

My current mods are:
- VT N/A Stage 1 Cams
- Mac Prochamber O/R
- Diablosport Predator
- GMS Upper Intake Plenum
- Steeda U/D Pulleys
- BBK 75mm Throttle Body
- FRPP 3.73 Gears
- Street tires (Exclaim UHP)
- Bullitt Suspension Kit
- SVT Cobra Front Brakes