Karl I'm having similar issues with a SRH 454 target gray model.
I bought mine used as well. I had the most trouble with CCI small rifle magnum primers. I had my gunsmith order and install a new Ruger factory spring and still had problems with said primers in both single action and especially double action shooting. Next I tried CCI small rifle non magnum primers, Rem. 71/2, and winchester small rifle primers. These seemed to function in single action but still had an occasional FTF in double action mode but seemed to work in single action mode. Finally I bought and loaded up some Federal 205 primer, with these I have not had any FTF in either single or double action shooting but I have not shot more than a couple hundred rounds so the jury is still out. I wish someone made a stronger hammer spring for the SRH's. Good luck with your gun. I'm looking forward to hearing your results.