charging problem

[GhettoPop]

drving home from work and my battery light comes on a few blocks before my house, volt gauge drops. went home got multimeter, batt (only 2k on it)was reading 12.8 volts. started the car, batt light came on again, and was reading around 12.0 with the car running! lshut it down, et it sit, too busy to mess with it. started it up a few times the next day, charging fine at 13.8 (around there, i forget exactly). a few days later the batt light comes on again at startup, volt gauge lower than usualy, and u can tell the car was not getting enough power. shut it off and restarted and the car was running fine again. whats going on! didnt see any loose connections at alternator. alternator put in about 15k ago, but its a cheap $70 remanufacted piece of junk that replaced the previously installed remanufactured piece of junk that only lasted 8k. gonna put in a 3g, but need a quick fix, any ideas?