The first time that happened to me was the second day I owned my bike. I was in a sweeper at about 95 when it started to "porpoise" on me. I thought I would have to have the seat surgically removed from my butt!! It also has a pronounced wobble when breaking hard into a tight turn. People kept suggesting stearing head bearrings but I pushed and pulled and shook but could not feel any looseness. Since I can't afford to replace the the front springs and fork brace at this time I have been living with it. I found that trail braking when it started (to unweight the front end) helps. But just b-4 Vogel I had it jacked up with both wheels off the ground and moved the handlebars just off center and they snapped back to center! And I don't think Yamamaha uses self centering bearings !
I had the same problem on an 82 Maxim that I bought and had to fix before I ever rode it. It seems that the races developed divots where the ball bearings sit. I went to Yamaha and the ballbearings were around $3 each and there were, like, 20 or 30 of them! So I ordered 3 and the races. Then I went to Ace Hardware and found the same size ball bearings for 15 cent apiece. I used those and rode that bike another thirty thousand miles with no problems. I assume that the bearings on the Venture have the same problem and I plan to repalce them soon.
Sorry this is a little long winded, but try getting the front wheel off the ground and move it just off center. If it snaps back to center and stays, then you might have stearing head problems.
Before the site crash, I think I remember something on here about replacement bearings that are pressed and not ball and race. If anyone knows about them or where to get them, please let me know.