have you changed anything in the geometry of the bike? moved the forks up or down, messed with trail (fork swap? clipons? ride height?)
the monsters are a little twitchy out front to begin with. highway speeds, combined with wind gusts, may make the front end float a bit.
happens to me sometimes too, usually wind is involved... but i've also decreased trail a little bit by doing a 999 fork swap, thus making my front end a little less stable than stock, but it has GREAT turn in.
i'd venture to say it's NOT your steering head bearings... but if it's a '94, i imagine the mileage is up there. you may want to look at replacing them whether they're the culprit or not.
Also, check this thread out as well:
http://www.ducatimonsterforum.org/index.php?topic=52708.0