Korg Poly800/EX800 Users

Hey there, I recently bought an old poly 800 recently. My problem is 
if I play on it (or just leave it alone with the power on) for more 
than 15 minutes, the display starts flashing `11 P', it then allows 
you to play no more sound but you can hear this wierd clicking in the 
background. HELP!!! whats wrong with it???!!!? is it the battery or 
something, I got the original power supply going into it, but no 
batteries right now, This has all made so me so sad! The sounds are 
so cool on this synth (I`m coming on to it from a sucky yamaha SY35) 
and I want my new baby to work....please someone help, will be dead 
gratful for any advice. Cheers :-D