'67 FB, 289, Holley 600, mild CompCam, headers. Set timing approx.12-15, runs like crap(especially under load and higher rpm's), set timing around 30, runs great. Question is "has the timing chain slipped a tooth or so"? Any insights?
when you set the initial timing(@idle), did you disconnect the vacuum advance? If not, you probably got a bad number. Probably want about 12-14 initial(with vacuum disconnected), then 34 total by about 2800 rpms with vacuum connected.
Something to look at is a vac leak. That can make the motor run poorly at low RPMs as you have a weakened idle signal, while getting better higher up as the increased air velocity overcomes the leak source.
I would do all the usual tricks for vac-leak checking to rule this out.