I doubt it is the problem with the early drivers. Those were a problem, but when the tender wheels and cups are clean, and the drawbar wires are making good contact all the way through to the engine post, it should run really well most of the time, with a possible little pickup hiccup over a turnout.
Do I understand you correctly, that the headlight flickers just before it slows down, and then it runs slow, and then speeds back up and runs normally? If that's what you meant to describe, this sounds like a partial short somewhere.
Things I would check...
1. Unhook the tender, flip it on its back in a cradle and touch leads to the two frame halves. See if it runs normally, or if the
problem is present. If it still had problems, then the issue has nothing to do with pickups.
2. Check that there is good separation between the two frame halves - especially at the rear and front. If seen some where
the frames bend inward at the bottom so closely that they short out to each other. They might be REALLY close and intermittently touching as it runs.
3. If the frames look okay, and you see this problem on its back, pull the shell, remove the motor and check that with leads
After you check these things, keep writing back and we'll keep narrowing it down.
