They are perfectly reliable double action but can bind when you try to thumb cock them for single action. One is a LNIB 3rd gen nickel detective special and the other is an e nickel Diamondback (that I just sent to Colt).
If it's being caused by what I think, it's a fairly simple fix. I'm guessing the cylinder bolt (stop) is disengaging itself from the rebound lever prematurely and popping back up to lock the cylinder in place before the hand starts to rotate it.
I studied the SP101 carefully before I gave up and sent it back to Ruger, and could never see any obvious cause for my problem. It left me with a bad taste for Ruger revolvers. What galls me most of all is that it went back THREE times and they couldn't fix it. I guess in those days they didn't care about customer relations and never offered me a replacement.