Hmmm... A Mark2 with a Mark2 sled? The fact that you were tinkering with the player, testing various things, makes me think it is a contact on either the player or the sled. The early Mark2s had trouble with the pins (for lack of the proper term) coming out of the wiring loom on the sled. Perhaps you have a loose contact there? It would mean pulling the sled to find out, though...