Create_direct API fails after some calls when Load test is done

Hello

I am performing load test for 1000 users using JMeter.
I found that this API

BaseRoutes.Channels.Handle("/create_direct", ApiUserRequired(createDirectChannel)).Methods("POST")

is the first one to break and shows Gateway timeout Issue(504). Though it was executed successfully sometimes during test but later failed and successively other APIs started to fail randomly.

When i removed this API from JMeter API lists, the load test was performed without any failure( no API calls were failing in JMeter).
I am using mattermost 3.4 version.

Can someone suggest what could be wrong.

Note: when the same test was performed on version 3.0, it was a success.

Any help would be appreciated.