You mentioned that you have a 2343 that runs fine. Would it be possible to move the 2343 rollers and pickups to the problem engine and see if it still sparks? That would clearly eliminate the pickups and rollers as the source of the sparking (though your previous work seems to have done that).
If the rollers/pickups are OK, then the problem is in the truck axles or gears providing too much rolling resistance or in one of the motors (or both if a maintenance item, but you seem to have covered most of those).
Though it would involve soldering, if the pickups are not the issue (as it seems from your previous replacements), then you could move one power truck from the 2343 to the problem loco. If that stops the sparking, you have isolated the problem truck/motor assembly. If the sparking continues, replace the other power truck/motor assembly. That should definitely isolate the problem to one power truck/ motor assembly. (From the amperage readings you recorded my guess is that the resistance is coming from an issue with the truck bearings or gears. I do recall that the long shaft with the gear goes into the end housing with the ball {it is there?} a certain way. Some of the more adept repair technicians might be able to help with this.)
If the trucks check out fine, at that point, you could remove the motor from the 2343 truck (which is working fine) and try it on the problem truck to isolate whether it is a truck or motor item. If the problem is in the truck, the easiest thing might be to pick up another truck assembly at a train show. If it is a motor problem, changing the brushes might help. You could also pick up another motor at a train show.
This is a huge amount of work, but I think it would allow you to isolate the problem and get to a solution.
Good luck!
