In cases where there's a lot of resource competition and you're trying to do large batches, you can see response times spike like this, but it shouldn't be happening as a routine i...
We don't have an open form submission API. This method will probably work for you, however: http://developers.marketo.com/blog/make-a-marketo-form-submission-in-the-background/
Are you trying to call this over http instead of https? That would cause failure, though I don't think it would cause this failure. Maybe you've gotten prod and sandbox creds mixed...
1. Only custom activities can be written directly with the REST API2. http://developers.marketo.com/rest-api/endpoint-reference/lead-database-endpoint-reference/#/Activities/addCus...