consistency starts and ends with the knucklehead wearing the brain bucket
That said, if the car goes from A to B pulling from the hit of the throttle to the stripe without a hint of an issue that's half the battle if not, you need to fix the mechanical issues first.
The other half is up to you. Start by leaving the hood closed between rounds, meaning don't change, adjust or screw with anything.
Make sure water temp is exactly the same every pass, burnout duration is exactly the same every pass, launch RPM is exactly the same every pass, shift points are exactly the same every pass and you are putting the car into the beams at exactly the same spot every time.
Are you observing and recording weather before every run and observing the flag?