how old is the ESC?
has it taken any hard hits/tumbles?

I used to run Sky RC in all my cars but found that over time, every single one that I ran in my off-road cars would run into problems with big air. I started using Kyosho gel tape to help absorb the impact... but nothing seemed to work for me. First I stopped running SkyRC in my 1/8 cars, then eventually had to quit the brand for my 1/10 off-road cars too. Never had any problems with my on-road cars since they aren't subjected to rough impacts and jumps at the off-road track.