The Ithaca NID design is actually much more "modern" than has been indicated. It has a rotory bolt locking mechanism that both simple and extremely strong.
I have 5 Ithaca Classic Doubles that have tens of thousands of rounds through them collectively and are all functioning perfectly.
I use these guns in all kinds of weather for both hunting and clay targets.
If you're having a problem, send the gun to Gunter Pfrommer. He knows these guns better than anyone. He will get it done right the first time and will not brake the bank.

Gunter Pfrommer
2954 Hopkins Rd
Rocky Mount, VA 24151
(540) 484-5555