the last time this happened to me it was a broken belt...
but it is a symptom of your alternator not working so if
you do not have a broken belt..or loose belt or sliding or slipping
up and down alternator... if none of these are your problem then
replace the brush holder/regulator in the back of the alternator..

and if you find this to be expensive just soldier in some new brushes..

you should always carry a spare of these in the dash...