Found a replacement trigger on the Numrich website and it was not to terribly expensive - paid 20 dollars for it. The new trigger arrived and it was installed. Definitely looked different than the one that was in the shotgun. Took it to the range and test fired it. No problems. Then I shot one round of trap without any problems. So it appears the trigger was indeed the issue. Thanks to all for your comments and suggestions.
Best as I can determine the shotgun was manufactured in 1966 and (to me) seems kind of interesting that 47 years later a problem shows up and it was possible to fix.