yesterday i was running my 1/8 truck and out of no where the steering was locking up on me, it would vary back and forth and then finally wouldnt steer. It would turn one way and then lock into place? Is it that the servo is bad? I already changed the batteries and that wasnt the problem.

sounds like the servo is bad but more info would be nice
are there any binding issues with the steering
what kind of truck is it?
were you ruinning it in water? or snow

Disconnect linkage at the servo and you'll be able to pinpoint the problem. Either servo and linkage on truck.

If screw holding the arm gets loose, you can strip the servo insert in the servo arm and servo will get off center and steer to once side like that. Take arm off and see what's going on. If teh insert is stripped, make suer you get the right one (for corrct number of splines on that servo) when you replace it.

Sometimes you turn it left, it would not turn, and suddenly it turn to the left by 'itself'...?

You probably have a cut wire that toughing each other (the black and red wire) or the connecting wire to the servo is loose, check them.

maybe one of the gears inside the servo has stripped, is it metal geared?

I think is the servo horn loose.