All good advise above. Perhaps try the gun assembled with the stock off and you might see what's happening. I would coat the hammers with a black sharpie and see if they rub inside the frame.
I would also have the original springs put back in the gun since the over powered springs did not solve the problem. The heavy springs may set off a chain of events that will cause additional problems. Primarily excess wear to the bent and sear nose.
I'm not a gunsmith but I've been keeping my old doubles going for a long time. I bet you'll find the answer and it will be something simple. Please let us know since someone else may have the same problem.
