I have a '99 Ford Windstar, the instrument needles intermittently go up and down

[ihadnuttin2dowitit]

real fast. Please help.? This happens real fast and last for no more than 3-5 seconds the goes back to normal.

 

[idontgivafork]

a couple possibilities. 1 is a bad ground, 2 is an overcharging alternator,due to bad cables or a bad voltage regulator,3 your backfeeding into the instrument cluster, and four the alternator is bleeding ac voltage into the system(could even be a bad fan etc)