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I suggest you unscrew the "lock" screws and leave them loose, unless you plan to transport the scope. Most people report the same - they just push the scope back out of collimation - yeah, they shouldn't, but they often do.
Screw "in" the primary mirror collimating screws until those springs are well compressed. That is, pull the mirror back down towards the fan end of the tube. Then try collimating again?
At what elevation do you collimate? 45 deg elevation is a good compromise. Once you have achieved fairly good collimation at that elevation, check it out as you move the scope from horizontal to vertical. In the ideal world, it will not change. In the real world it probably will for these scopes! A mm or two won't kill your viewing. Often it is the primary mirror "sagging" on the springs, hence the recommendation of getting them under more compression or even replacing them with stronger springs.
But an 8" mirror isn't that heavy - are you sure your upper tube assembly is rigidly attached to the "mirror box" - all screws on the struts done up firmly? Any movement of the UTA relative to the mirror box will be a problem. You cannot feel any twist or tilt or strange behaviour in the UTA as you elevate the scope?
Also, better that you collimate it where you are going to use it. My 8" solid tube was great. I could bounce it for two hours to a viewing site and the collimation hardly changed from the last session - but a strut design, I would not be surprised if moving it around upsets the collimation. Do you collapse it to take it outside, then open it again?
Also you are right, from warm house to cold outdoors won't help your view.
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