Electrical problem

I have a 1999 Ford F150, 6 cyl with std trans.  At times, it won't start.  I turn the key to the on position and the gauges jump to the max, then go back to zero.  If I try to start it, the starter sounds like it may be trying to engage but the motor wont turn.  I used to be able to turn the key on and off several times until the gauges didn't fluctuate and the thing would start.  Now I turn the key on, and nothing.  I can hear a rely clicking behind the fuse panel but I can't start it.  If I wait until I get some lights on on the instrument panel, I can stast it by coasting and engaging the clutch.  It runs fine- just won't start.