medici,
A thought for you,
The full 90 degree backswing comprises of two parts (forget overswinging for now)..
basically your pivot/shoulder turn turns your shoulders to about 70-80 degrees from where your shoulders were parallel (ish) to the target line at address...
The final 20-30 degrees (to make the 90 degree look) is caused by lateral movement og the shoulder blades...the left shoulder blade extends AWAY from the spine and the right shoulder blade compresses TOWARDS the spine....normally this is not thought about, but it is well worth checking this action (to get the correct feel at the top).....
If you stand with your hands up in front of you in a praying position (upper arms parallell to the ground), then remove your left hand, then (without moving your shoulders or chest) move the right upper arm (humerus) to the right by 90 degrees. At the same time, notice how the right shoulder blade compresses towards the spine and notice how the upper right arm is not tucked into the right ribcage.....note this is a lateral movement, not a raising movement...
I get the feeling maybe you are turning to 90 degrees and then any additional (not realized) s/blade action (which you classify as lifting) is adding to that, hence ther across the line effect...
Try just turning to about 70 degrees and then working your shoulder blades and see what the clubshaft condition is at that point...
I am in sunny Devon BTW....great weather here at the mo, rain and gales for a week....