ORAS. If you switch out before Slow Starts 5 turns end, does the counter reset? And if the 5 turns are up and your stats return to normal, then switch out, does slow start activate again?
The Counter does reset, as it does when you complete the 5 turns of Slow Start, it'll always reset when you switch out.
http://bulbapedia.bulbagarden.net/wiki/Slow_Start_(Ability)
Attack and Speed is halved during the first five turns since sent out
Slow start will start again each time you send Regigigas out Source