On Thu, 14 Jul 2016 11:53:30 -0400, Phil Hobbs wrote:

Too small an output cap, or too low an ESR? Three terminal regulators
can oscillate. Try hanging 100 uF aluminum electro on the output and
see if it starts behaving better.

The datasheet says output caps are "optional" although I do always use
'em. The one I'd fitted was 1nF which seemed a bit small, so I subbed it
for a 100n but it's made no difference. 100uF seems HUGE! I might try it,
though.