While driving at night, have you ever noticed other people's headlights that appear to be shaking and bouncing around? Isn't it annoying and distracting? Your brain is so good at noticing motion that it's hard to ignore those bouncing headlights, especially if they're in your rear view.
Embarrassingly, I found that I was one of those "other people, bad citizens" when I replaced a headlight bulb and noticed the reflector was loose.
Step 1: Open hood and check reflector
Follow the instructions in the manual for replacing a headlight bulb. The bulb screws into the reflector, so you can feel if it's loose or not.
This is 1996 Volvo 850 GLT with cold weather package. I imagine the headlight assembly and surrounding components aren't that different for models around the same age.