Simon,

those steppers have a torque of probably 0.4Nm @ 2.5A. You're running them at best at 1.2A so you'll get maybe 0.2Nm static torque. At 240rpm this is reduced to 0.1Nm as per my previous post.

With the frictional inefficiencies of drawer slides and threaded rod you'll be lucky to get any movement at any useful speed.