"I suspect that the problem with the now occasional inconsistent cycling from forward to neutral to reverse has to do with the connection between locomotive and tender."
Arnold - As I mentioned in my earlier comments the sharp bend in the cord caused my #618 to derail, and when you look at the latest pics you've provided perhaps the bend causes your fragile 4-pin connection to come loose and cause other problems as well. Just a thought.
Secondly, I know (by you mentioning trying to run the engine through the F-N-R cycle) that you've been running in conventional mode, but did you realize that this engine is command equipped - or is supposed to be (see first page of Owner's Manual). I only mention this because I see your cord has 4 pins (which mine originally had when I bought it new) but I wasn't into Command Control as yet. Later, after I had had someone re-wire mine to correct the derailing problem, at some point I noticed that they had used 3-pin wire - but really didn't think much about it at the time. As a result, when I eventually got TMCC components and then tried to assign an ID# to this engine, to my dismay as soon as I put 18 volts to the track my engine went 90 mph and I was very fortunate to be able to catch it before it went flying off my layout table. Consequently I concluded - rightly or wrongly - that you need a 4 pin connection in order to run this engine in CC mode, but a 3 pin connection will enable it to be run conventionally. In my case I've been content to run my #618 in conventional mode only.
