My ignition timing is going crazy, very unstable

I trued setting the ignition timing today to 2.5 deg BTDC as per factory but the mark was running all over I.e. Very unstable. I set the thing the best I can and took a long drive, after about 50 miles of driving the engine start pinking badly at anything above 1/3 throttle. I suspect the timing went crazy and advanced by itself.

What could cause this? The spring inside the distributor? Is it hard to take the distributor out?

Thanks

Syed