more like insufficient output from the alternator. could also be a really bad ground...

what you're experiencing is when you place any type of load on the alternator, its having trouble coping. you could be looking @ a number of things. of course... your alternator could be failing. however... it could also be the rectifier. if you have a multimeter.. .or no someone who has one... i'd suggest using it to check resistance in both the B+ side of the charging system as well as the ground side.

B+ Side... set the multimeter to the ohm setting (omega symbol). but the positive lead on the B+ POST on the alternator. this is usually the largest cable, which you can trace back to the battery (POS) terminal. place you negative lead on the battery post. do not use the cable itself or the terminals for readings... since you're bypassing a point of possible resistance. report back w/ your findings... high numbers (above .3 ohms) indicate high resistance...

anything beyond that will cause you to have issues getting the rectifier to adjust the voltage input for the alternator to handle the load

Ground Side... again, w/ the multimeter on the ohm setting, place your positive lead on the negative POST of the battery. then, place your negative lead on the alternator's housing. this will tell you if there is high resistance in the ground side. look for readings above (i believe) above .1 ohms to inticate a ground problem.

these are simple tests. to determine a possibly simple problem to fix. high resistance could be loose connections, corroded connections, broken cables, or even just a corroded battery terminal. these are all far easier to fix than replacing an alternator (cheaper too)

hope this helps... i'll check my numbers in the morning to make sure i gave you the correct resistance specs... good luck