Thanks Aussies!
The other Merkel problems I have seen on other forums (even including a new rifle which would not fire one of the barrels) have all turned out to be Merkel factory rifles, not Merkel shotgun conversions.
As you likely know, the Merkel factory has made shotguns for many different brands over the years, but all the guns were of the same basic design and features. This includes the Simson Co Suhl shotguns which have a good reputation for strength and reliability and are known to make good donor guns for double rifles. The Merkel shotguns are essentially the same as Simson and also make good donors for double rifles.
Why do I care and glad those troublesome doubles were not made from shotguns? It is that I am planning a .458 caliber double rifle based on a Merkel 12 gauge shotgun and am sensitive to the names Merkel and Simson(of which I have heard only good).

Sorry you and others have had problems with Merkel double rifles, but glad they were not based upon the Merkel shotgun.