possible errors might be:
-Cueing across the line of aim and making the CB deflect slightly on it's way to the OB
-Wrong perception because of wrong eye position
-Bad grip that leads to steering (not enough of a cradle)
Maybe it'll help to read and try what I
wrote in this thread:
http://forums.azbilliards.com/showthread.php?t=299755
On Dr. Dave's webpage there are a lot of videos that might help you too:
http://billiards.colostate.edu/
one of multiple videos you'll find here:
Dr.Dave/grip and bridge:
http://www.youtube.com/watch?v=PevddF2gRh4
Dr.Dave/vision center:
http://www.youtube.com/watch?v=680o8EChP_o
...although BoB Jewett steers to the left when he shoots the straight ball in too at 2:20 min

(just where he wants to show how it is done correctly) cueing across the lign of aim to the left, making the CB slightly deflect to the right).
As he does not shoot very firm, and his cue shaft does not deflect a lot and the unintended left side English is already gone
when the CB makes contact with the OB, the CB does not rotate anymore although he makes contact with the cue ball around an 8th of a tip to the left of center ball imho.
It might only happen because of his elbow (upper arm) that goes slightly to his right (away from his torso) during his final stroke motion)
Nevertheless the video explains very good what you have to be aware of!
Maybe this videos of mine concerning grip and fingers or the video of Andy Segal will help you too:
grip:
http://www.youtube.com/watch?v=Fq3lCuScYWM
Notice: I sometimes tend to steer slightly to the left on very firm shots as well and am still working on this after more than 25 years of playing...
Andy Segal:
http://www.youtube.com/watch?v=p3QgH0fl1kE
Last hint: try to film yourself while stroking from the front and from behind!
best of luck to you!
EKKES