I have a similar problem when I debug a 800x600 AGS game I'm making. It'll freeze the game's screen, but it will flash between it and the current screen. It happens when my laptop is unplugged as well.
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I checked for something modulised but couldn't find anything.I must be crazy then. Oh well.
That gives me the error saying I can't use the change room command within non-blocking events such as the repeatedly_execute_always one.A-hah. Well, I didn't have the manual in front of me at the time... but Christian has come and save the day.