Opened up my troublesome DX5e yesterday, as some have I also also found the slider did not seem to be allowing the switch to travel all the way to either the 'on' or 'off' positions, the 'off' position might not seem to be a problem but the 'off' contacts seem to be involved in the trainer port, so some of the circuit is different when using it as a buddy box.
Something interesting I saw on the 'off' contacts, I was ohming the critter out and saw a dozen ohms across one pair of the 'off' contacts and 200 on the other pair, after working the switch directly a few times with the slider off this dropped to 1-2 ohms.
I think this shows an important component of the failure, that being full travel of the switch is critical to fully wiping off the oxide buildup on the contacts, probably when you set the switch to off or on the contacts actually travel a bit past their rest position, but when restrained by the slider slot the contacts just bulldoze crud up to bu not past their rest position.
Another aspect I also found is that if the switch was not fully in the 'on' position, the long switch arm restrained from the last bit of travel, the entire switch arm is 'rocked' back a tiny bit (tilted towards the 'off' position), so not only are the contacts not fully in position, but the spring pressure on them is actually reduced because if the arm is cocked, so is the actual slider in the switch. In this condition the degree of contact was very susceptible to very small motions of the switch arm.
So it seems like the problem is a perfect storm of crud buildup and reduced pressure on the contacts and the arm being able to transmit tiny motions right through to the contacts, all due to the slider not letting the switch achieve full travel.
What's funny is the hole in the plastic looks adequate, but the thick plastic decal actually has a slightly smaller hole.