You state that the engine is ringed. Could it have a free floating Dykes ring that has rotated to have the end gap on the intake side? If so the engine will have almost no compression when turned forward, but will have the same old compression when turned backwards. If it is conventional ringed, one hot (lean) run can take the temper out of the ring causing compression loss. If that is the case a new ring is in order along with another long rich break in.