2008 250exc-f speedo won't turn on/stay on?

[bexi]

Hi all,
I have just bought a ktm250excf 2008 model.
It's like brand new but has been sitting for 12 months so i had to clean the carby real well as it was so badly blocked up.
It's all up and running great now with a new battery.
Weird thing is the speedo.
I've read other peoples posts about the speedo's failing to which most of them say they need a new speed sensor.
The weird thing with mone is that the actual speedo works, it's just that the display won't automoatically turn on or stay on. The only way i can get it to work is to hold down the main button. As soon as i let go of the main button, it turns off straight away.
If i hold down the main button while riding with one hand, the speedo does work as it should.
Anyone got any ideas on how to help?

thanks
bexi