I have work around my problem.
So that I try with below code, and It's work at both IE7 & IE8.
It's look like a double focus but you can replace the focus event
with another event some kind like show alert messenge, focus other control, then finally
will be work.
"var comboBox = document.getElementById('"
& cboMaDichVu.ClientID &
ScriptManager1.Page = Page