No acceleration, lights crazy then kicks in & we go?

I have a 1996 Z71 Pick up truck 4wd. At any moment (usually traveling 50-60 mph) the speedometer will begin dropping to zero.  The dash lights will light up leaving the ABS light to stay on.  You have steering control but when you press down on the gas, the engine raves up.  Then all of a sudden, it will ‘kick’ in and you continue driving like normal.  Sometimes it will immediately correct itself while other times, you best pull over and wait for it to ‘kick in’.  Yesterday, when it did it the speedometer would not work until it ‘kicked’ in.  It may to it today but not at all tomorrow.