it may be ALOT of different things.

-battery might be going...maybe...get it checked out at Advanced Auto or Auto Zone. Those types of stores run free tests.

-battery cables could be going though I think you'd notice all the INSANE amounts of corrosion and the fect that there wouldn't be much battery cable left.

-alternator might be going out on you. I know that after I replaced mine a couple months ago, my headlights/interior lights started surging and backing back down. I've got too much voltage at times...and you have the opposite. My suggestion would be go to a website like goboom.com and get a refurbished alternator (if that IS the problem). I wouldn't buy a new factory one because they cost 263.62 (I checked just the other night)


Hope that helped some.