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This could be an episode of syncope which is another term for collapse. This can be a sign of a few things but could be a heart arrythmia. This results in weakness, stumbling, this lost look, etc. (My lab has an episode every 3-4 years). While this could be life threatening if the arrythmia does not correct itself, these can be very difficult to diagnose as a routine EKG is not likely to pick up any abnormalities. Other issues that are possible are focal seizures, adrenal gland issue, etc. I would try to get video of an episode (cell phone cams are great for this) as this can be valuable to your vet. It may also be worth having your vet take a good look and have bloodwork and EKG done. I hope this helps and gives you direction.