You're probably running into the standard problem of too many volts over too much time.
Servos weren't made for the type of pain which we put them through.
If you want to improve the longevity of your servos, keep them down at 6.0V, which is what they're rated for.
If you do chose to overvolt the servos, be prepared for the types of things that you've experienced.
Even at 6.0V, you'll find that you still run into the same sort of problems (although less frequently, and less drastically).
Again, it comes down to the servos not being made for this type of wear and tear.
If you have the kind of money to spend on two servo arms, you might think of purchasing one arm with better servos, rather than two weaker ones.
When you overvolt a motor of any type, you emphasize both it's strengths and it's weaknesses.
Any flaws in the manufacturing of that motor will become more prominent with the more voltage and strain that you put on them.
If you're going to continue to overvolt them, I'd suggest frequent periods of rest.
You can also try cooling them down every so often with ice-packs from your freezer (the soft, non-leaking, variety, of course).
Be sure to wrap the ice-packs in a paper towel just in case of minor condensation.
To be honest, overvolting should really only be done with toy RC car motors.
Those are a couple bucks a pop.