New poster here......
I've got a 4" stainless 629-1 that is 100% reliable with mid range loads, but over rotates the cylinder (jumps timing?) with factory 240's or equivalent handloads. Ends up between cylinder notches about 80% of the time with these loads. I know this was addressed in later versions of the 29's, but is there a reliable fix for these earlier models?

This is not normal that a 629-1 can't shoot factory 240gn loads.

I feel the cylinder stop and spring need to be inspected. Possibly a new spring will correct it or the stop is not rising high enough to hold the cylinder from jumping. When was the last time the gun was internally cleaned? How do the cylinder stop notches look?

Visit the revolver section and look for the FAQ sticky. I put stuff in there which may help you take the revolver apart for a better look inside. Good luck.

Thanks for the comments; I'll both look at the FAQ and inspect and maybe replace the spring. I do a complete disassembly only about once every five years since this one doesn't get to the range much anymore.

Smith addressed this problem in the 29-4 ( I think) by replacing the cylinder stop spring with a heavier version and with a beefed up cylinder stop. You can order a heavy duty cylinder stop spring from Wolff for about $4 Wolff springs or you can call Smith customer service and ask for the new spring - I think the heavy duty springs from S&W are red in color.

Excessive endshake can cause this too.