Get a better radio or get rid of the Savox. Problem solved. Savox are one of the most power hungry servos you can get. That and their QC sucks, so sometimes you get one that works great and doesn't draw a huge amount of power and other times you get a crappy one. Its a roll of the dice, and the price reflects that. Also my understanding is the older 2.4 modules compound the problem.

Something to also check to help reduce the amount of power the servo needs is your endpoints. If the servo hits the limit the wheels can go but the servo wants to go further it increases the power drawn. This doesn't seem to be the issue though since its doing it on straightaways as well. A higher C rated battery may help if you are running a lower C rated battery. What it comes down to is something is drawing more power than the rx can supply. Whether it is because of a general lack of power supplied or too much power being drawn. Better Tx and Rx can handle more power supplied to them. Better servos draw less power. Its not always just one thing. Sometimes its a package deal that's the problem.