When does it give you this feeling? Braking? All the time? Just with bumps?
I thought head bearings too. If they're worn or 'dinked' from being too loose too long, or way too tight, they'll try to settle into their 'low spot'.
I've not had this, but I could imagine if one leg was binding it would cause that feeling. Maybe try the 'old zip tie trick' on both legs and see if they're both moving up the same amount. Just wrap one (or two to make 'em long enough) just tight enough to hold it's position around each tube. Slide 'em down to the dust cover. Go ride. After you stop (the important part), measure how much each has gone up.
Preload the same on each leg? Same heights in the clamps? Legs straight and aligned?
Worn tires can cause this too. I had that problem, among many others, when I got Vrooomm. It'll probably be easily visible as one side of the tire being more worn than the other. If you have to look at it hard to tell, it's probably ok. If you look at it and say "Holy Crap!", that's it