12.5-13.0 A/F does sound fairly normal; did you ever run it any richer than that with differing effects on the detonation? I know we're hashing back through stuff that was already brought up, but I don't want to have to search back through another 6 page thread.
EDITED: misread other details...
And yes, EGT=Exhaust Gas Temperature...I don't think it's quite as simple as higher = leaner, lower = richer, but when you combine the readings with A/F or EGO info, you can better figure out what the engine needs.