You never forget your first tankslapper.
As I'm sure you've discovered, there are many causes. You need to find the cause of yours.
There's something wrong with the bike.
Fork oil (absence thereof), unequal rear snail adjusters, worn swingarm bushings, loose or totaled stem bearings, tire balance, worn lowers, tubes dropped too high in the triples, loose upper triple, axle bearings, twisted frame, misaligned fork tubes, loose axle nuts, etc. etc.
For future reference, the way to handle a tankslapper (especially on a bike that's strange to you) is to push forward equally on the bars and slow down while it's still in the minor "speed wobble" category. If you wait til it enters the tankslapper category you may fall down, die nice.