Time Out Error while sending IDOCS to SAP

While Webmethods pushes IDOC to SAP , we randomly noticed the following

com.wm.lang.flow.FlowException: pub.sap.transport.ALE:OutboundProcess Timeout

platform details
IS6.0.1 SP2
SAP 4.6 SP5 with Fix 1 and Fix 5
WIN 2000

I cleanup the mailbox using sweepTRX service which is schedule to run daily once. The Xtn.log and xtn_audit.log in Wmpartner/config are 25M and 21M right now.