so you are driving 5 volts through a cable (likely several feet long) into a coupler through another cable and into a pi a device... that sometimes needs up to 2 amps, and you wonder why it's having power problems?
i think any article you can google up regarding line loss is going to have your answer.
in a nutshell you are trying to drive a very low voltage along a very long line, through several connections (each having it's own small loss introduced)... with a small device like a mouse or keyboard that would consume say 100 milliamps , this wouldn't be a problem. you may not thing a connection is that big of a deal, but your adapter has one...the coupler has 2...the pi has one... couple that with the large load the pi requires, and the problem is exasperated manyfold.
this is precisely why the electric company uses high voltage in the wires that run to your house, but step down the power before it goes in the house.
what you should be doing is using a higher voltage supply (say 12 volts @ 2 amps or so) and running that a half mile from the plug into your control panel then using that 12 volts to power a DC-DC buck converter to get your 5 volts to run the pi. even if the cord from your 12 volt supply is 100 feet long and the voltage at the end had dropped to 7 volts because of how long it was...you can still get 5 volts out of it. not much you can do starting out with the 5 volts you need and losing it from there.