Dune racer upgraded to 18v won't sustain power 19

[Teambleach84]

Hi guys, I've taken the winter to overvolt this awesome dune racer my wife picked up for Scammer. After much time reading threw this forum I got It wired up with the bigger 775 motors and all seemed great. The problem I'm having is the runtime is odd. My boy jumped right in and it was fast but after 5or so min it starts to really slow down until it won't run at all. Maybe 10 min total. Thinking this was odd I checked the voltage of the batteries and they were not close to being dead. Furthermore after waiting a couple minutes power would be restored and we could drive it back to the garage. So I'm guessing maybe the pedal switch gets too hot or something in that vein. I did not run new wire should I have?

[Teambleach84]

Picked up three 6v sla batteries from interstate batteries. I have a 30A fuse between them all. I've seen people talking of rewiring everything with thicker gauge. Is that necessary?

[Biggjimm]

Could it be the motors are getting hot???
Just a thought.
Good luck. Jim.