So when you hit a straight line cut, the material warps away from the torch, or the torch retracts away from the material?
The AVHC is a very simple operating tool, and they all operate the same. I have a question as to what the amperage voltage and speed settings you are running at, because if it hits the straight lines coming out of corners or Arc's, it will accelerate to your Program Feedrate, which is probably pretty high. When the machine accelerates the voltage number changes, and the AVHC is simply responding.
This problem would be universal on AVHC systems because they all operate the same. My guess would be that when the machine hits a straight line it accelerates to the program feedrate and the set voltage is no longer accurate due to it's new faster speed.