Easy: timers end after they're captured by OnEvent_Timer or when they're stopped. So, when you need to repeat it, add a new timer!
code:
function OnEvent_Timer (checker) {
if(checker == "checker") {
_clone (ChatWnd);
MsgPlus.AddTimer("checker", 5000);
}
}
A good tip: always check if the timer id is the right one.