Have you searched the forum at all?..some common problems you have. Answers are here..

In the UK speedo's are in mph..I can get up to 90 mph easily enough(but why)
without any instability BUT the bikes upright riding position,big wheels,tyre patterns and suspension are limiting factors..

I weigh 14 stone and am 6'2" ..Kev's fork mod has stiffened the front end up nicely and the rear shock is set up stiffer..tyre pressures are vital too...