I don't think this is necessarily a servo or battery issue. It may well be, but from the description of the issue, it sounds like there may be a slight maladjustment of the throttle linkage causing the carb not to be completely closed when returning to idle. A stronger servo will help, but I expect you can get it working properly with the stock equipment (which would be a good idea whether or not you plan on upgrading). Always first recommended RTR upgrades are the RX battery and the steering servo (throttle can normally be left til a bit later, you don't need a particularly strong servo on the throttle, especially if you have a decent battery), as the others have correctly said. Truggies especially need strong steering servos, it makes a big difference to how they drive.

PS Also worth mentioning is that after a fast run, the engine is supposed to 'high idle' for a few secs, this is not in itself a problem.