Nintendo has released a troubleshooting guide for people experiencing problems with their Joy-Cons.
The official Nintendo support page is aimed at Switch owners whose Joy-Cons are “not responding” or “responding incorrectly.”
Many reviewers testing out the Switch before launch complained of problems with the left Joy-Con dropping out during game play. We had the same problem and noted it in our Nintendo Switch review, having it happen “relatively often” during play.
Among the steps are making sure the Switch has the latest system update, but also checking for other sources of interference like wireless headsets or even microwave ovens. Also, don’t put it near an aquarium. That’s a suggestion.