Due to the complex Colt action, unless we can see it, it's tough to offer much help.
This could be caused by a deformed lug on the ejector or it could be a burred or narrow locking notch on the cylinder.
My first move would be to order a copy of the Jerry Kuhnhausen shop manual on the Colt revolvers.
Volume One covers the Python and all the older style Colt's.
This shop manual was written as a training manual for new gunsmiths.
It shows Colt factory repair methods and is the best source of Colt revolver information there is.
Using the manual should help you diagnose the problem and the repair.
The main use of the manual is to properly diagnose the EXACT problem and eliminate attempting to repair something that isn't the problem and causing even more trouble.
This manual is money VERY well spent for the Colt owner.
If you don't feel up to attempting repairs, you could loan the manual to a local gunsmith.
http://www.brownells.com/.aspx/pid=2...-A-SHOP-MANUAL
As for the brass dust and scratches on snap caps.
This could just be the soft caps rubbing on the harder steel frame, or there could be a burr on the breech face of the frame.
Examine the face of the frame, especially around the firing pin hole for any burrs.
I would suspect it's just the snap caps.