Regarding the control of blank instruction screens, it requires integer values for the time, e.g., (d1)--that may be why the trouble is occurring. If the point is to create a visual break between the questions, you might find the onset (o) parameter to be useful. You can use this to delay the question wording on any given item, and you can specify the value in milliseconds.

See: http://www.empirisoft.com/medialab/h...ters.htm#onset