Thanks, guys, but as you'll see in my last post above, the problem was solved simply by lubricating the roller axles. I love it when there's such a simple solution!
And how did I come to learn this little gem? I had the same problem with a diesel engine. It would stall on switches. I checked continuity and everything seemed ok. In checking my rollers I had noticed that they were not spinning freely so I put one drop of oil on its axle and wiped up all excess. That was 2 years ago but since then I lube every roller with just one drop of oil. If you look at the old Lionel manuals they stated to lubricate the rollers. I guess they knew what they were talking about 
I always lube the rollers, maybe why that issue hasn't gotten me. 
I still have a couple that the spacing of the rollers causes issues over switches. Since they both happen to be PS/2 locomotives, I can just add the wire internally to the tender from the pickups on the tender, job done. The power comes over to the PS/2 boards, and there's even a convenient empty spot for a second center rail wire, I slip a contact into that. I also recommend a PTC in the 4-6 amp range in series with the added pickup wire to guard against derailments shorting the rollers between the tender and engine.
