Then in late 2009 I started having shoulder pain. Seemed to be somewhere in the the delts, seemingly not rotator cuff though sources say you can't tell the difference. I could actually push my finger against a sore part of the delt down in the muscle.
Finally this year I think I've identified the cause. Not a rotator cuff at all but a delt strain cause by doing the exercise wrong.
Using light weights I had learned to lead with my hands as I raised my arms; this is mechanically undesirable and strains certain bundles of muscle fibers deep within the medial deltoid that are involved in twisting the humerus to spread the hands as well as raising the humerus.
Instead I need to lead with my elbows, to have a sense of using my upper arm and shoulder and not the lower arm and hands in the lift.
This has already let me lift heavier without pain.