Chris,
Sorry for the delayed answer. Sounds like your pack is not able to handle the load. The surge is more then likely the esc tripping.

By lowering the throttle, you are not lowering the amp draw, just the amount of time, that the esc and pack are seeing the amp draw.

I'm guessing here, but I'd say your pack is borderline to the amp draw you have, it's able to handle the amp draw in very short bursts, but can't sustain it at wide open throttle.

Hook up a meter to your set up, and you should have an acurate answer real quick, as if the pack is the culprit.