Quote Originally Posted by avguy View Post
When you rechecked the timing marks, did you check to see that the harmonic balancer and the keyway looked good? I know it doesn't seem very likely to happen, but a balancer that somehow disengaged or broke away from the keyway after those first 4 minutes, and is no longer turning in synch with the crankshaft, might certainly cause all of the things you're seeing.
I see petrified.rabbit addressed this, but when I removed the accessory pulley to check the timing mark, everything looked good with the keyway.