Novak HV-4.5 issue

[JThiessen]

I just installed my replacement HV 4.5 system (the first one was replaced under warranty - never got to drive it more than about 100 feet).
With fresh charged batteries, the new one runs great - cant keep the front wheels on the ground. However, after about 2-3 minutes of running, it drops off enough that it appears the batteries are completely dead. A check on the batteries shows they still have 7.XX volts ramaining. It does this on all batteries. I'm geared at 72/14, and 90K in the diff's. Any ideas?
Its now been a year since I first bought the system, (and due to the fact that this a hobby for me, I only get a few times a year to run them), I still haven't been able to run a complete battery set through this set up.