QTP stops execution with no error

Hi All,
I am using QTP 11.
QTP executes the second step below and Stops execution all of sudden.

This happens intermittently not always.

Have anyone faced similar issues?

Please suggest me a way out.

SiebApplication("Vodafone Qatar ST3").SiebScreen("Contacts").SiebView("My Contacts").SiebApplet("Contacts").SiebList("List").ActivateRow 0
Wait 1
SiebApplication("Vodafone Qatar ST3").SiebScreen("Contacts").SiebView("My Contacts").SiebApplet("Contacts").SiebList("List").DrillDownColumn "Last Name",0
wait(2)
SiebApplication("Vodafone Qatar ST3").SiebScreen("Contacts").SiebScreenViews("ScreenViews").Goto "Contact Detail - Accounts View","L3"