It sounds like the card may have been a bit loose for a while and with the kit off and cooled down, the resulting thermal contraction may have been enough to break the connection between card pin and main PCB socket. As you say, reseating the board has sorted it out.
I own an 82. I know how the cards fit - and I struggle to see how they could come loose. They are - in my experience - quite difficult to seat correctly.
Maybe they were never seated right…? You did check the other channel…?