I just drove around all summer and fall wondering why my engine felt out of balance. I had already checked the balance shafts, and it seemed like it was never totally consistent. It always seemed to vibrate more with higher RPM, but it usually seemed worse during WOT. It was so bad at times that I would never take it above 4500-5000 RPM. I had pulled the #1 plug, and confirmed that the timing mark on the pulley was indeed still lined up in the correct spot.
Well this weekend the harmonic balancer let loose completely, and started eating my lower timing cover. I immediately replaced it with a spare I had laying around, and holy smokes! I can't believe how fricken smooth this engine is now. Totally kicking myself for not trying that sooner.