At 243,000 my LT1 has started to misfire and buck. I have totally replaced the entire ignition system including the PCM. I had no codes and with all the changes now it shows code 43. At 60 mph the spark advance is 30-34. When it starts misfiring it drops to 16-19. At idle the spark is 19. At 60 the engine is running good. I have replaced the knock sensors, ignition module, Electronic spark control(ESC), Optispark, and finally the entire computer. The knock sensor on the OB1 should read yes or no but it give a number. Any ideas what I'm missing?