When I charge my computer the cursor is not operating properly

Hey guys. I have a problem. While I am charging my Macbook Pro (2015) the cursor is trembling and its performance is really bad. I took it to the Apple store in Bristol, where they changed the trackpad and the battery, but the problem has remained there. I went back the next day and they told me that they were really busy(!!) to accept again my computer that day, despite I was travelling abroad in a couple of days and having already cancelled my ticket, since they were late to give my computer back. After talking for long hours in the phone and replicating the problem myself, something that they couldn't do in the store, I sent and they proposed me as a solution an extension cable, which really doesn't make any sense. Does anybody of you have any real solution that will save me from carrying around a more than two metres long cable all day?