OK.... as others have said, it really does sound to me like the XL is overheating. As the loco has a separate sound module, that would explain why the sound is unaffected while the motion and lights etc all shut down.
Can you put an ammeter into your power feed to the track (not across the two feed wires, but just into one wire so it is in series with the power feed - I know that should be obvious to you and probably to most other folks on here, but I just mention it for the benefit of anyone else reading this who hasn't learned the basics of electrickery!) and see what it reads (a) with loco and tender running, (b) with loco only.... if you could also make up a dummy plug for the tender to connect its pickup wires to its motor wires, so that you could run the tender on its own (under analogue of course) then you can check the amp draw of the tender motor alone too.
Both motors should be running at round about half an amp each, at most.... if either one is drawing significantly more, then that is most likely to be the source of your problem. If they both seem to be OK, then I would think it's going to be the decoder - thankfully, as you've got the separate sound system, replacing an XL isn't too bank-breaking.
Jon.
