E3D temp drops as soon as it reaches set temp

I just purchased a E3D v5 Hotend and when I attempt to heat it up to 290c as per the instructions, it gets to 290 but then seems to shut off. I am using the Sprinter firmware and a Sanguinololu board. this has worked fine in the past using a JHead Hotend. I have tried setting the THERMISTORHEATER value to 5 and 6.

Any ideas as to what may be the issue?

Thanks,

John