Well. Spent an hour today with the lead mechanic at the dealer I purchased the 260saw from and had the place stumped also. We found a few odd things though and all point to a bad BAR. All diagnosis was done with the side cover off and nuts in various stages of tightness:
1. When we flipped the bar around we couldn't replicate the problem.
2. When we only finger tightened the nuts enough to hold
an adjustment, there was no problem. Tighten them down, bang, it's back.
3. When we turned the chain in reverse it would ride out of the groove on the rim, always to the inside.
So it appears that there is a twist in the bar, not discernable by eye (there wasn't a surface plate around...) A new one is on order, and we'll se Friday. The're ready to give me a new powerhead if this doesn't work.
Thanks again for all the help.
John