black rain, I posted a brief reply under your question in the Smith & Wesson section before I saw your post here.
Although the spring on the cylinder “lock” or stop seems OK it is almost certainly the cause of your problem. Under heavy recoil the cylinder stop is moving enough to allow the cylinder to unlock and roll backwards. It is a common problem in the more heavy recoiling M-29’s but can happen with heavy loads in M-25’s as well.
Ether replace the spring with a new one or take the old spring out and stretch it a bit to add strength and that should cure the problem.