x reg alto speedo problem

[slatts]

hi i bought my girlfriend a suzuki alto a few months ago and had loads of problems. recently the speedo has been acting up it works but sometimes goes funny like when were driving about 30mph it will jump up to 70 or 80, it only does it every now and then but u lose track of what speed ur doing and at 60mph it goes off the clock

can anyone help

thanks