Dang! Well, with this smaller scale stuff, you don't really need HV servos, both front tires are under 150g total! (I think it's like 58 grams for each whole front wheel)! For the bigger scale stuff, HV servos would probably be a no brainer!
I've heard that HV servos on a normal non-HV receiver, are power hungry. It might have been you who said that actually! My servo is a normal, non-HV one, and I'm running it off my trusty DumboRC X6 RX.
Ok, so you tired all the other things I said, like going somewhere else (because there may have been radio interference), ect? I'm sorry to say it, but maybe you're gonna need new servos. Did you get the car new? How long has it been sitting in the box? Also do a quick search, see if others in...
Dud you try the other things I suggested? If you did, and it still twitches, I'd say get a new servo! Make sure to get one that will fit, and has enough torque/power/speed!
I just had a thought, can you run an HV servo on a normal voltage receiver? I would guess so, but at lower torque and...
Ok, so you've connected everything properly (I take it you also made sure to plug everything into the right channels), turned the radio on first, then the receiver, ect. try doing the same test somewhere else, there may have been radio interference from another appliance, unlikely, especially...