Beyond something obvious you could see wrong with your steering rack, a very loose servo saver, or some serious binding in your pivot balls, there can only be 2 causes for what you describe, stripped servo gears or stripped splines in the servo horn. If you are using the 2 stock servos I doubt you have stripped servo gears as it would require both servos to have the same gears stripped in the same places for it act that way, but since they are both working on the same rack it's possible they both have gears stripped in the same places. Give the entire rack assembly a good looking over for cracked or broken/worn out parts, make sure your saver nut has not backed off, and make sure your pivot balls are all free. If you don't find the culprit pull the servo horns and see if the horn splines or output shaft splines on your servos are stripped, then check the servos for stripped gears. If you don't find the problem after all that, I'd say make sure your radio rig is not acting up.

As has already been mentioned, you would be better off with a single high torque (at least 200 oz/in) metal geared analog servo for steering in your revo.