Engine off vs engine on, buggy sitting on stand - amp draw is the same. Buggy running on the track, amp draw higher.

I would try another batt borrowed from someone who has proven it works in his buggy - or better yet, get him to run your battery and see if it works like its supposed to.

If you've tried everything, then what's left is to start swapping out components with new ones cause one of yours is not right. Beyond that, if everything is new, I don't know what to tell you.