performance degradation on battery

XPS 17 9720.  Severe performance degradation on battery - sometimes.  Under warranty.

Sometimes, charger is plugged in the connector light goes out and things start to misbehave.  happens with 2 different dell chargers. Unplugging and re-plugging resets it.  There is clearly a gremlin in the power system.

I ran the hardware quick test from the boot menu, time:

plugged in:  5:41

unplugged:  17:12

that typical?

windows and dell set to balanced power settings.  Battery health is Good.

Maybe someone can give me a pointer to help troubleshoot to fix or to get service or warranty service? 

FWIW, my agent experience:

Agent had me run hardware tests, said, "We thoroughly examined the hardware, and there is no defect in it that could be causing the problem." went to performance options, changed the setting to "adjust for best performance", and said let us know if that fixes it.  ??? hmmm. 

So i said that can't be acceptable, this thing should support those settings, i even reinstalled recently to check? so mic drop: "Based on the issue, the next best action is to perform operating system reinstallation. ...  Let me know when you are ready to proceed."  Ouch.