I know a guy who wasted three inteks like this. Finally figured out the motor plate was flexing, or too light, or something of that nature.
Looks like block stressed from un even mounting plate.BENT OR TWEAKED ENGINE PLATE WILL DO THIS. TIGHTENING AN ENGINE DOWN TO A BENT OR WAVEY PLATE WILL PUT UNDO STRESS ON THE BLOCK. ADD VIBRATION AND BOOM.
Had one do the same thing,from uneven mounting plate.