Updates.
I finally got the spare time and all the needs for thorough track cleaning. I used cotton swabs / q-tips, first doused in isopropanol, to rub the track surface. The q-tips got "pretty dirty", but I thought to see them get even more "black" from dirt.
Then I run the bigger loco, 88973, few rounds with a speeds little over ~100. After that "drive-in" I tried slowing the speed in steps, ~90 and ~70. The results were
better than before, but the stuttering is still clearly there and notable. And doing some more of "warm-up / drive in" laps didn't improve the situation, it just revealed a wider view to the jerkyness and stalling. Some rounds go "pretty smoothly", but in most it seems like the loco is twitching itself forward - never going steady or smoothly. And then there were still those few times, that the loco just stopped, like the stalling would be too much for it to bear. It also got stuck 2 times so that the controller was unresponsive to the loco. I had to physically lift the loco from the track and then put it back before it would "woke up" again and start to take commands from the control unit. And sometimes even that didn't work on the first try.
Everything under the speed of ~70 is totally off, and the loco stops eventually, usually somewhere between "immediately" and "after 15cm of track".
So.. now the track is so clean that there's no spot that would leave a stain to a white cotton swab. The brand new loco, 88973, have not been runned no more than total of maybe 6-7 hours in these tests, and it has factory-clean wheels (although tried carefully cleaning them with a q-tip, but nothing came off), so it should run smoothly, or at least
smoother than it does now. All the joints on the track layout were double-checked while I was cleaning them. All the wiring is checked again.
Also, the smaller loco (81569, from the starter set) really seems to be officially dead now. As I mentioned on my earlier post, it had became very unstable responsing to controls at all. Now it "doesn't even try", there's no sound, no twitch, no microscopical movement - even when looking it without its top casing. I tried numerous times to get it running again, but it just sits on the track totally unresponsive.
I'm now uploading couple of videos I took while making tests on the clean track, I'll update the links to this post as soon as ready.
And again, this thread and community help has been a huge aid, so many thanks to all the helpful people for that!

All tips and advices are still welcome, but I'm really starting to think that it's best to send the kits back, as they're new and they should work and function properly.
[UPDATED]Links to video clips:
<< Video 1 >> Running forward, for this clip it actually went the laps without the
worst of the stalling, but if you keep audio up and look closely, the continuous twitching movement can be seen (and heard) as the loco moves. Could be seen better if my had would act as more solid stand for the camera..
<< Video 2 >> Running the loco backwards, the base noise it makes is multiplied, it's actually quite loud whirring noise when the loco moves. A short clip, catching first a lot of stalling and then ending to one of the 'unresponsive to control' -situations that occurs.
<< Video 3 >> Running the loco backwards again. In this clip the unsteady running, twitches and judder is much again visible and easy to spot.
<< Video 4 >> More of the unstable running. See next and the last video for what happened in about 5seconds after I stopped filming this clip..
<< Video 5 >> .. and resulting a stuck stop again. As a side note; although it might appear like a pattern that in these clips the train seems to always stuck near the power input track / on the same straight portion of the track, it's not. It does that randomly on the whole track area.
Edited by user 30 January 2013 12:01:19(UTC)
| Reason: More updates