Viper's gear different than most other 13.5T's due to the lower Kv rating
my 13.5 is geared at 25/84 in my SC10 and 17.5T is 32(33)/72 in my B4.1

try 33//75 in your Buggy with the 13.5
Check temp after 2 mins than again at 5
My motor comes off at 140 after 7 mins