For the love of god

[Lizard]

That would be 65mph calculated with 3.7V per cell and 3.82 inch tire diameter.

Sounds still a little high for 4S, but then the Slash is very lightweight and has a lightweight drivetrain. Also the GRPs are only 350g for a complete set. I guess that car is around 3Kg.

I wouldn't have thought that such a setup would make them die like flies.

Maybe try some extra capacitors and make cables as short as possible and use good quality connectors (if you're not already doing that anyway...)