OK – I double checked the 68Kohm resistor in the feedback loop, and it measures at 1.5Kohms on my multimeter. The stripes say that it should be 68Kohms, and I’m 90% sure that it measured correctly when I checked all the parts before soldering up the eyelet board. It must have become damaged somehow during assembly.
I’ll buy some new 68Kohm resistors tomorrow and see what happens if I replace it.
As always, thanks for all the help!