I had the same problem with a coupe that had been stored 12 years. It would start and run OK, then start running rough, with an intermittent knock, and lose power. I discovered that the timing was changing and that it would be all over the place, even a few miles after I retimed it. When I pulled the the side cover and started the engine, I could see that the timing gear was moving fore and aft with every change in RPMs, which explained the knock and variable timing.
Next, I pulled the front cover and I found that the thrust plunger was all the way back and that the internal spring had rusted and broken into three pieces. The short story is that I replaced the thrust spring and all was well. An unthrusted camshaft will tend to "walk" back and forth and you will never be able to keep it timed.