Motorcycle Repair: Wobble at high speeds, axle nuts, good peoples

Question
I own an 82 Honda 900f supersport with 18k miles on it. When I first got the bike it ran fine,now it has developed a severe front end(handle bars and wheel) shake at high speed over 80mph; not just a shimmy,but back and forth action.The bike continues going straight but the front tire and handlebars won't stop the side to side until I drop below 80. I'm not sure of when it began.I do all my own mechanics. I bought two used tires from a friend.They are on the bike now.I had them balanced at a Honda dealer.Most people say it's not the tires.A few years ago I changed the fork seals,one side still leaks a little. My bike got pushed over once with a 70lb load on it while it was up on backstand.I can't think of anything else that would change the way it runs.Buff

Answer
Ok used tires? not good. peoples lives depend on those things... yours! dont ever do that again. put the bike on the centerstand, and weigh or tie the back down so the front is in the air. take a pin or something, spin the front wheel and see if its bent, very common. then check the bearings for looseness. check all the lower end bolts and axle nuts for loose. follow the tubes up and tighten the 4 clamp bolts. THROW THOSE DAMN TIRES AWAY.