What motor are you running? I noticed the same thing, and thought for sure my LiPo was fading. I'm pretty certain it turned out being heat in the stator core that was causing the issue. I put one of Novak's heat sinks on my motor (I run Novak 13.5/10.5) that forces on to the stator, and the problem went away.
With all that said, the Orion packs have considerably lower voltage than the others. If you're seeing that kind of difference, when you're ready to get a new pack, you might consider something else. The heat thing has been proven time and again. You can make it happen every time with a $10 heating pad.
And the last thing to consider is voltage. If you get a battery with more voltage, you're going to need to gear your motor down to account for it, otherwise you'll generate too much heat. If you're right on the edge with your gearing using the Orion 4800, you're almost definitely going to need to make a change with any other battery, as its voltage will be higher.