Gator with 20v dewalt shutting off in high gear

I have an old peg perego gator that I found on the side of the road. Wired up one of those cheap amazon adaptors for a dewalt battery but my son has the need for speed and when he switches the gator into the high gear, it runs for a few minutes then dies. After a minute or so, it’ll move again but what am I missing here? What’s the simplest way I can make this thing go fast and not cut out in the middle of driving?

Thanks