Your servo gears are making noise suggesting that they are under quite a load. I would check what has already been said and free up those ball joints. I did mine by bending them off the balls almost to the point they pop off, holding them there and then putting them back. Not had a problem since.
If you are looking for a servo, MKS servos are probably the best there are. Very few people use them in cars however.
I have one in one of my aircraft and although half the size of what you would run in a Mardave, has a lot of speed and an insane amount of torque considering its size. The only way the case can take the loading is because it is machined from solid billet - the whole thing.
In my Mardave I run a Sanwa ERG RZ. I firmly believe that the fastest servo you can get is the one you want in a pan car. High torque isn't required. I've looked around for fast servos and the fastest I have found -
http://www.mks-servo.com.tw/goods.php?goods_id=hbl980
I must say that although the Hitec analogue servos do not centre brilliantly, the digital ones I have have all been great and still work very well. I am not sure it is fair to tar all of the range with the same brush, the analogue ones are cheap and good for applications that don't require the precision of a direct-link pan car steering.