G6E TURBO!

Hey guys, I'm new to the forum and I've got a little problem. I've got a G6E turbo that's been lowered with superlow king springs but has the original shocks and struts. It also has brand new tyres all round. When I'm drying on the highway 80km and over the car feel very floaty and sways from left to right ?. Even over little bumps the back of the car seems to almost jump a little from side to side. What could this be in your opinions? Thanks for your help guys, much appreciated.