Charging problem

This is on my 85 Ramcharger.

the problem started when i jump started someone.
it has a 318 2bbl 727 and is 4x. still has the lean burn and is pretty stock.

what is happening is it seems to be dying. its very sporadic. the night i drove home after giving the guy a jump i saw my lights dimming, i turned them off and road home on the KCs (figured it would draw less current then the headlights, running lights, interior lights etc etc) even the stereo was cutting out.

i went out to start it in the morning and it crankes slowly. like a dead battery.

however it had about 12 volts in it (multimeter check). the alternator is putting out 13+

i dont know if its my alternator or voltage regulator.

the battery is a sears die hard weather handler (south version) i took it into sears to see if it has a bad cell and when they charged and load tested it came out fine according to them. the other day i drove it for about a 1/2 hour parked it was inside a store for 5 min and it was to low to start (11 volts) {yes i keep a multi meter in my truck} a friend came and jumped it and it was fine the rest of the day.

this is confusing the crap out of me so any help would be very appreciated.

confused in Carson City, NV