Why is my script editor comming up with connection invalid after running it for a while?

This is the script I've found on the web I'm trying to just have the script drop items for me by pressing 1, but it works only for a little while before script comes up with an error "error "System Events got an error: Connection is invalid." number -609"

I use this script

delay 3 tell application "RuneScape" to activate repeat delay 5 set mydate to current date repeat while ((current date) - mydate < 1) tell application "System Events" to keystroke (key code 83) end repeat end repeat