Suzuki GSX 1100 E

Hello, Suzifriends! Since May of this year I have been the owner of a GSX 1100 E, Bauj. 87. The machine now has a driving performance of 60 000 km and is in good condition. However, I have a problem: when the gas goes down, at approx. 80 km/h, very strong steering vibrations are detected. Free-hand driving is not possible – there is a constant feeling of uncertainty, which was never the case with other motorcycles. And at high speeds in long-drawn cure The machine swings up and there are dangerous situations. To meet this, new steering head bearings were installed, re-tired and balanced and looked after on track of front and rear wheel. The problem persists. Now I would like to know if this is a known lack and how to meet it.