you need to have 3 control sectors linked together - three squares. The left square is the floor/cieling hieghts of the block at the highes point of it's cycle, the middle square is the hieght that it starts at, and the right square is the one that determines the height when it is at it's lowest point. Tag the middle sector to a linedef on the right square(the line that borders the middle square), and give that line the effect, "activate floating platform". Then, take one of the non-bordering lines on the middle square, and give it the FOF effect you want, and tag it to the sector you want the FOF to be in.
And that is how we make vertically moving FOFs! Any questions?