Was the patient stable? What was the BP? Any vasopressors? A third degree block will not have the same PRI on the one consistent normal sinus beat that you mentioned that occurred just before the funny little beats (FLB's). You may be looking at things too closely or too hard. Sometimes an explanation is unrealistic, such as a heart rate of 20 to 30 on an alert, oriented old lady with a systolic bp of 180. I have seen that, which defies logic.