Success! If anyone is curious, going back through all the adjustments did the trick. I hadn't done a very good job of adjusting the hook timing before, it was off a bit, also the feed dog was high. After those changes, I set top and bottom tension very light, started with T70 thread / 18 needle. After a few seams slowly increasing bottom tension got a nice stitch. I increased and decreased SPI a little beyond the range I plan to use. Then I went through the same exercise with T75/18, T90/20, and T100/20 adding only a little top tension when needed. All produced a nice stitch. At this point the top tension was near max. Then I tried T135/23 which maxed out top tension to get a good stitch. On a lark I tried T210 top, T135 bottom and it stitched pretty well through two and four layers, about 1/4 inch, though that's clearly maxing out this machine.
Bottom line: Thanks to leatherworker.net, Constabulary for confirming the snag problem and previous posts, and also Eric, whose previous technical posts I followed closely, this Singer has gone from being stored for years in a barn to back in sewing shape in my living room -- I love this forum!