Sorry about the late reply, anyway...
You need to use a timer. An easy way is to use GM's built in timers, "alarms".
You can put that code you have in an alarm event and at the end do something like:
CODE
alarm[0]=30 //30=1 second
You will need to start it by using it in the create event as well.
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You could also make your own timer.
CODE
changeTime+=1
if changeTime=30
{
//put your code here
changeTime=0
}
There's a bajillion ways to slow it down, these are just a few.