wM 7.1.2 I assume you still on legacy world 
Anyways coming to this issue, are you using DMZ/RI port that talks to Internal Server and when you say resource handle have tuned the Resource settings on your IS to handle the receiving volumes?
Also if it is on the RI external port then try to tune the Private Threadpool Configuration and also adjust the Backlog ,Keep Alive Timeout 600000 settings (both on RI and Internal IS port settings)
However, I do echo with Holger’s suggestions in moot 100%
HTH,
RMG
#webMethods#edi#Integration-Server-and-ESB