we recently had problems with the LT API, which powers languagetool.org and the add-ons (Firefox, Chrome, Google Docs), being not very responsive and often returning errors. These issues should be fixed now and languagetool.org and the API should be faster and more stable than ever. If you're interested in the technical details, please check out this issue.
Still, our server is a single server, if it crashes, languagetool.org and the API are down. The only viable solution seems to be to switch to Amazon AWS and run at least two machines with a load balancer in front. We're currently on a more traditional hosting company (Host Europe) that doesn't offer load balancing.