Approximately how many rounds through your 66?
After 10 years my 66 started misfiring, naturally it started at Winter Range. A local gunfixer tried the new mainspring and firing pin with no luck.
A pard suggested checking the headspace and gave me some old feelers to grind down. Sure enough about 0.015" excess.
Ordered some new links that were long enough to take out that excess headspace. Works like a champ.