I have similar behavior on my Baja with an XL2 as well. I'm going to be adding a cap pack and an external BEC this weekend to see if it clears it up. It's not quite as bad as this one shows, but it's very noticeable, especially about 5-6 minutes into a run.
I will update with regard to improvement after the change.
Baja 5T - Castle brushless conversion, every other hop-up known to man... 8S LiPo power.