Nikon D70 Night Shots

I am shooting night High School Football games and using the Sport
Digital Vari-program modeon my Nikon D70. In the day this mode works
beautifully. I could not ask for more.

At night, I might get a set shot where there is little or no movement,
but all action shots are blurred.

Can anyone help with this? I know it is a lighting problem. I just do
not know what adjustments I need to make to catch full action football
shots. Do I move out of the Sport Vari-program and into something else
and then start changing the ISO...etc

Will