PSP Buttons not always working....help!?!?

[lee-ds]

Hi,

A friend of mine has a psp which until recently was working fine.
One day last week he switched it on but the x,o, buttons etc wouldn't work during play but start etc did, he tried it with another game (tennis) and the buttons worked but start and select etc wouldn't!!!

Any suggestions on a cure would be greatly appreciated

Cheers in advance

(not been dropped or watered btw)