Hi again Ranger
Well, I can't say why it's happening (but I can guess) without looking at it, but from your answers I can however tell you what is happening. If my explanation seems over simplified, please understand that it's not my intention. We simply don't know each other, and this is the internet after all, so I can't know how familiar you are with revolver part terminology.. hehe
So anyway, when you unlatch and pull out the cylinder for loading, you'll see a spring loaded piece inside the frame pretty much directly above the trigger. That is the cylinder stop. It should be spring loaded, so that you can push it down a little with your finger, and it will return to its normal position when released. When the cylinder is closed, the stop should in normal function push upward into a groove in the cylinder (the groove of the chamber opposite of the one which should be lined up with the barrel). When you cock the gun, or begin pulling the trigger in double action, the cylinder stop is pulled away from the cylinder allowing it the freedom to be rotated by an internal part called the hand. When the hammer is cocked, the timing of the gun should have rotated the cylinder into such a position that the cylinder stop's upward spring loaded pressure pushes it into the next groove on the cylinder.
That being said. What is happening on your gun is that with the hammer in the cocked position, the cylinder stop is for some reason being restricted from returning into its "freedom" to raise back up toward the cylinder. Actually, you can intentionally create this occurrence on a normally functioning revolver if you draw back the hammer only a little bit only to the point where where the cylinder stop is being pulled away, but is not yet free to return, the gun will allow the cylinder to spin as you're saying.
So, what does this mean ?
Well, first check with the gun's cylinder open if you can depress the cylinder stop, and if it will return when you release it.
Situation 1: It does not depress and return. Then the cylinder stop spring spring is simply most likely broken. Annoying, but very simple fix. BUT if that were true, the cylinder should technically spin regardless of whether or not the hammer is cocked.
Situation 2: It works normally thus far. Close the cylinder into locked position. Visually inspect that the cylinder stop is properly engaged (basically, can you see it, or is it pushed down into the frame) If it is visibly enaaged, cock back the hammer , while watching the cylinder stop. Does it disengage (down into the frame ? Does it re-engage. If so it should be partially visible, and released to "push" against the surface of the cylinder before the cylinder has rotated enough to allow the stop to fully engage into the groove.