That sounds like clutch slip. What happens is that when you floor it, there's a little boost spike and a small spike in torque. This overcomes the clamping force of the clutch and it slips a little, but then it clamps again.
To test again, try 4th and 5th gear, flooring it to try to make a boost spike. Don't do this too much or else it will glaze the clutch. Try to gently roll onto the gas pedal to prevent this spike and it should help a lot. What mods do you have? The more mods, the more likely clutch slip.