I agree with Henchman's diagnosis of the bells and whistles so that sort of leaves us with why it's happening.
One thing I would like you to check please. When you hit this problem, look down, backwards and left, you will see 2 black handles, these are your throttles. They should be set to auto (the middle), make sure they are.
I just had a thought you (or a command on your joystick?) are either pushing the throttles into full power (and damaging something not showing on a panel) or they are below auto, and thus don't do anything. I don't know why it would wait until the same point, unless at that point your doing something else and catching the page up/down keys?