Help Please Electrical problem

ok i own a 1987 pontiac 6000 160k miles and basically ive been having some very weird electrical problems at first like a year ago after starting the car the headlights and tail lights would flash on and off and the volt gauge would peeks and come back down so i figured it was the voltage regulater in the alternator so i replaced that and the battery...i replaced both of them not more then 4 months ago but not that its gotten cold out the car will not keep a charge it will start up fine on most days but when i get to where im going it wont start back up...after leaving it sit for about 15 mins it starts back up some of the times...but lastnight after letting it sit for over night it wouldnt start back up sometimes it trys to turn over and other times its completly dead...if i try to jump start it not even 15 seconds into the jump it will start up like nothing happend and is good for about a day or 2 and then starts acting up again...i have become completly stumped i dont think it could be the alternator seeing as it doesnt die well running only after i shut it off...it seems that the cold weather plays a role in this problem too but i am not completly sure...any insight into this problem would be very much appreciated thanks for your time