tallyho, If the cylinder locks up properly when cocking slow, that tells me the timing is fine and there is nothing wrong with your hand. The bolt (cylinder stop) may be rounded a bit or it could be as simple as a weak bolt spring.
First, check the cylinder's lock notches. They should be cut sharp with no peening. If the slots are peened, you will feel a ridge on the trailing edge (opposite side from the leade). These can be dressed and tightened up somewhat with a hammer and punch. This is a common problem with the Ubertis because they use soft metal in their cylinders. If the cylinder slots are sharp, then I would suspect a worn bolt, weak bolt spring, or a worn slot in the frame where the bolt is located. If the previous owner liked to cock the gun briskly, the massive cylinder will slam into the bolt and cause the slot to peen wider. This makes the bolt drift and not lock up properly into the cylinder.
You can check the "bolt slack" by cocking the gun and checking for excessive side play. A little "wiggle" is OK but too much will allow the cylinder to spin past the bolt when cocked. If you do have excessive side play, try to determine if it is coming from a wide frame slot or a rounded bolt (or peened cylinder slots). Remove the cylinder and grab the bolt with your finger nails by straddling the top strap. It should have no more than a few thousandths of side play. While you're at it, push the bolt down. It should have considerable spring tension with no free play.
Let us know what you find.
