Help, please. Serger needle thread breaking.

I have a White 534 Serger that has been on the shelf for 20 years. It wasn't working well when I put it away so don't why I thought it would self-heal sitting on the shelf. Anyway, I have threaded and re-threaded and it continues to break the right needle thread. Told a long time ago that it was out of timing and I took it to a repair place and it came back just like I took it in. I am such a scrooge that I can't believe I would have to just pitch it and buy another. Anyone with a suggestion. I have new needles and new thread. Just cleaned it up and oiled it. It has almost always broken the same thread.

Christi