G5 start up problem

When I start my computer it freezes on the blue screen after the apple logo spinning wheel screen.

Ive tried unplugging.reset the button below the ram slots,installed ox tiger from disc to a second internal harddrive (same freeze up).

I can get past the freeze up by starting up while holding down the shift key.

I hope someone can help.I live in Dsllas and will take it to the apple store at willowbend mall monday if I cant get it working.No warranty so I hope I can fix it first.