The Traxxas 2018 is a standard torque, plastic geared, no frills servo. If you have had a crash or two you might have stripped a few teeth off one of the gears.
I suggest a "high torque, metal geared" servo. This will end your problems permantly, as well as give you more positive control over your steering.
You might try a Hitec HS605mg. These are a decent servo for the money.
There are many different brands and prices of this style servo. Any one of them would be a fine choice. It's all up to how much you want to spend.