Have the 1256/1258 servos in my buggy.

The steering one though seems to stop working properly after 5 minutes on my JQ buggy. At first I though it might be a dud life pack, so bought a new nimh pack to go in it. Charged it up and went for a run today, only to find that after 5 minutes the steering was gone. Is this the sign of a servo about to give up completely, or are there some other things I need to check? The steering is nice and free with no binding, so it shouldnt be under excessive load.

Any thing I should check, and can anyone diagnose my problem outlined above?

edit: I check the spacing and its got great clearence above the chassis, so it shouldnt be heat related either.