There isn't a lot of information in your post, but it would appear that your not the first to experience a problem in this area. See Get Email Preview Thumbnail and a prior post on Code@ Retrieving a thumbnail image of an email via API?
It is possible to make a POST to our API using cURL with the Authorization header. An example GET request with cURL would look like this:
curl -H'Authorization: Bearer OAUTH_TOKEN' https://www.exacttargetapis.com/platform/v1/tokenContext
If it's only working some of the time I would guess that the problem is related to authorization. Make sure that you're generating a new token every time.
If you need instructions on getting a login token which will let you go directly to the email page see Getting a Usertoken for Entity Direct Login
BTW. this a similar approach used by some of the ExactTarget productized CRM integrations as well so it's known to be working.