Happens to some of us some of the time. I dialed my timing light from Zero to 60 degrees and still MIA From Idle to 4500rpm it never showed it's face. I pulled #1 spark plug and applied some masking tape to a zip tie and wrapped the tape around the black zip tie. Inserted the tie into spark plug hole and made a mark on the tape where the tie met the header bolt. Turned engine by hand and notice the mark moved up..made another mark.. kept turning until the marks I made no longer went higher than the header bolt. Turns out my balancer ring has slipped nearly 4" Glad I found it before i slung it off and had it rattling around against the timing cover and pulley. Gen 1 sbc 350" That makes 3 that this engine has spun since i have had it . I suppose it averages 1 every 100,000 miles or so. Think your engine wants a lot of timing all of the sudden to run well. Check TDC.