I had one on my Fireball 225 V6 that would slip. It had a miss, after many hours of chasing distributor and wiring issues then struggling with the mighty single barrel carburetor. I noticed when checking timing if I reved it a bit the timing mark would change positions. I drew a witness line across the damner with a felt tip and discovered the slippage.
After that I’d set timing with a vacuum gauge. Now after discovering this service I’ll get it fixed.
Gosh, I love this site.