Since the crumbs are so small, the dog doesn't always know if they are there or not, but will keep on looking and sniffing. Start dog from this position. Gradually lower height of pause table until dog is starting from just the table top on ground. Finally, remove table top. This is very similar to those who have a dog that blows zones only at trials.
Intermittent reinforcement has a very powerful effect on behavior.
If you sometimes allow the dog to choose when to start or when to hit zones, you are sabotaging your training efforts. However, there are still many trainers who are very consistent with their enforcement of staying at the line, and who still have problems with the excitement of competition factor. This can be a difficult problem to overcome. However, if you have not been totally consistent with your enforcement of these errors, you have something you can try to improve your situation. Training alone can really contribute to problems like these.