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I've thought I've had drifting during guiding issues in the past, but I can basically put it all down to differential flexure in the system. If the guide scope and imaging aren't absolutley rock solid together, this will occur.
In some shots I've taken I've measured aproximately 7arcsecs shift between successive 5 minute exposures. 7arcS is not a lot in real terms, but when stacking it becomes significant.
Try doing a stack without aligning, and check out the path that a particular star makes.
As stated before , if there is rotation, you will see the alignment star as a dot, and the rest forming larger arcs as you mave radially away.
If you are not guiding in Dec, any polar mis-alignment will show up as lines in the Dec plane only.
If PEC or other RA issues then there will be a line in that plane.
If you get a curved path ( showing shift in both Dec and RA), I'd put it down to differential flexure between guidescope and imaging scope.
This caused by gravity effects bending components at different rates as you track across the sky. You may even find tthat while imaging in some parts of the sky, the problem appears to be not so bad, while in others it is terrible.
HTH.
BC
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