<I was at my local dealer, I'm broke and couldn't buy anything but did look at a Model 17 with an 8 3/8" barrel. I asked to see it, did my usual "timing and carryup" test.....very slowly cocked the hammer, 4 of the 6 chambers did not lock up on my "paint dry speed" cocking test. In slow DA, catching the hammer so as not to dry fire it, it locked up fine on all 6. On "medium" speed SA cocking, I found 2 of the 6 still did not carry up.>

I usually find that the slow pull DA is the best timing test. I'm listening for that cylinder stop to click up in the same spot on every chamber, just before the hammer releases. That said, you may have a gun that may fail this test, simply because the sear releases before the hand has completely carried the cylinder up. This could be a problem with the sear fitting, not the hand or ratchets. I'm kind of baffled by your observation that the gun in question carried up fine in slow DA, but 2 chambers would not carry up in single action. Thumb cocking the hammer allows the hand to carry up the cylinder higher than with a DA trigger pull.