Before we deduce that the radio set is the culprit, try the following few things-

+ Make sure the receiver antenna wire is complete and not cut or frayed anywhere. To be completely sure, run a continuity test from the end of the antenna to where it solders into the receiver (will have to pull the receiver case apart); should be 0 ohms.

+ While you are in the receiver, look for any sign of water intrusion or foreign matter.

+ Make sure the antenna is fully threaded in the transmitter

+ Pop the crystals out of the car AND the transmitter and verify that you get a nice positive engagement; on the receiver, use a ziptie to secure the crystal into the receiver. I usually also ziptie the servo leads into the receiver as well

+ Pop the servo horns off of the servos and make sure the servo teeth or the horn teeth aren't stripped- where it would grab sometimes but maybe not others.

+ Make sure the throttle servo and linkage smoothly moves the slide valve in and out of the carburator. If the linkage is setup wrong or at a bad angle, it can bind when going in and out and get stuck.

It could be a case of a bad RTR radio set. If I tried the things above, I would replace the radio set (TX/RX). It can be an AM set if you are on a budget. Typically for RTR 1/8 scale buggies, you could move the stock steering servo up the the throttle position and then get a decent metal gear servo for steering (Hitec MG645 very popular)

Also, if you are close to power lines or a radio tower or cell tower, those can cause interference. Maybe run the car in a different spot and see if it works any better.

Good luck!