Note that the coor dyna command computes the average structure over the number of frames processed. It will be very slow if you try to a read a large number of single frames from a long trajectory since it starts over reading the trajectory from the beginning at each invocation. You can use "coor read file ifile .... CONT" (io.info):
READ COOR FILE IFILE 10 UNIT 51
READ COOR FILE CONT IFILE 10 UNIT 51
will first read frame 10 and then frame 20 from unit 51.