What focuser does it have? Some GSO focusers have located under the drawtube two thumb screws placed in line: one is a tensioner ( the one closest to the EP) the other is the stop (closest to the tube). If the tension screw is too loose it will introduce slack into the drawtube. The stop screw will also act to tighten the system. My 8" GSO Newt has this dickie focuser too, and it still catches me out. Bloody nuisance, really.
A little jiggle of the drawtube with an EP in place will quickly show any slack.
Does this movement of the laser show up with the drawtube rucked in all the way & with both thumbscrews tightened? If so, & you've firmed up the spider & secondary, then the mirror cell might be loose, or the collimation screws wound in too far (ie, the mirror is up too high). The latter just needs the primary to be lowered closer to the housing. Might be that the mirror cell needs a bit of attention. I'm assuming the focuser proper is firmly secured to the tube,
I doubt it is tube flex. You wouldn't get so much movement in the laser if it was, as John says.