Here's what I did!
I created this image with a dark purple background and a soft brush stroke with 100% Opacity and 100% Flow. So far, the light looks nice, but it's far too intense for a background.
I've set the layer Blending mode to Linear Dodge (Add). Using this will preserve some texture from the background, if there is one. I've also set the layer's Opacity to 40%. And from this point those pesky lines are starting to appear.
And here's how I fixed it!
STEP 1
With the spotlight layer selected, Go to Filter > Sharpen. Do this twice.
STEP 2
Go to Filter > Noise > Add Noise and set it to 1%, Gaussian, Monochromatic.
Here's the difference:
