Powermac G4 Cube needs PRAM reset everytime to boot

Decided to boot up my G4 Cube after sitting many years, unused in my office.

It fired up but the clock was out of date, reset that but then after shut down it would boot up but the no monitor, keyboard or mouse.

If I manually press the PRAM button on the motherboard it boots fine, and you can restart the machine over and over again with no issues.

It's only when you shut it down that it requires a manual PRAM press to boot.

I have rpleaced the 3.5v battery as I though it was causing issues, I have updated the clock and date, done a software update to so it's running on OSX 10.4.11

Anybody any ideas of a possible fix?

thanks