I know this is rather an old thread, but I've got the same issue (slow starter after warm-up) and I've been doing research before digging into it to try and get a better idea of what to look for. I've read all 16 pages and all the diagnostics angles seem to have been hashed and rehashed so I won't ask any of that stuff, but I have a couple of questions for ya Kit.
1: Did you ever follow Hush's suggestion of crossing the poles of the starter solenoid with a screwdriver to see if the starter bogged down during one of the instances of the bike being in "failure mode?"
I know that you bought a new solenoid and after that starter performance improved for a bit then the problem showed up again (or at least I think that's how it went, the thread WAS 16 pages including more then one extremely informative, mentally side-tracking manifestos by helpful members
) so I'm curious to know if that step was ever taken.
2: I read that you changed the solenoid-to-starter cable with a new one and the positive battery cable with new, but how about the battery to ground and the other ground cables as well as the low-voltage wire leads to the solenoid?
3: Did you ever pull the headlight fuse to see how that affected things?
Seemed like the cliff-notes of this thread would read something like; The major components of the charging system were all changed and/or tested and everything checked out except for the field coil resistance which was sporadic in it's readings and could've been attributed to a few different components/connections
The starter issue manifested with three different starters, one of which was new/like new
all electrical connections having to do with ignition and power generation were checked and cleaned
Some primary power delivery cables (by that I mean the ones that handle full voltage/amps, wires leading to/from 30amp fuse ie: main power delivery through the ignition and fuse box) were changed/cleaned but not all (?)
That sound about right?
I really appreciate the effort that everyone put into this thread. It does seem like enough 650 owners are experiencing the same problem that a common factor would be suggested. On the other hand, these bikes and their components ARE 30 or so years old and in Kit's case it seems to have just been a bad cable or solenoid. I haven't looked for other threads on this yet (or even read the other threads Kit linked to in the first post as it took me 4 days to read this thread as I just don't have more then a 1/2 hour at the net at a time) but I plan on looking for more reports. But in the meantime I thought I would get the mental ball rolling... faster.
Thanks!
Will-