If I understand your problem, it sounds like the cylinder bolt is worn out, or the spring is weak, and possibly the hand spring also. The cylinder bolt is spring tempered and is forked. One tine rides over the hammer cam, and has to be fitted to adjust timing. If the spring is the problem, you just replace it. These can be found made from music wire these days, which is much better than the original spring. The hand spring is very thin and wears where it rides up, and down in it's frame channel. Many frame channels are quite rough, and have a case hardened contact surface. I worked on a third model once that had less than a box of ammo shot through it, and the hand spring was already worn in half. The channels need to be filed smooth with a diamond file, and polished to stop the wear on the spring. Since you say the cylinder locks tight in full battery, I would look at the trigger-bolt spring first, then the bolt, and hand. Also check the ratchet on the cylinder for excessive wear. None of these parts are terribly expensive, but if the cylinder ratchet is excessively worn, you may have to find another cylinder.