hmm,
thats kinda wierd, cuase,
if it works some times,, after you jump it then that means your alt is still putting out some kind of juice
and it must be hiting your battery,, so that means your fuse links are good.
it sounds like your battery is out, cuase you say some times u have to jump it and other times you don't, your battery could be shorting out your alt, to make it read wrong,,
a bad battery can make a alt go bad
if this is on your 80's model,, i woudln't know, i own a c4 and the way ti works is,, it goes from your alt to your starter to your battery,, thats how the L98's are
but it also sounds like your alt is going out,, so anyways,, here is how u can tell you need a ohm meter
touch the positive and negitive on your bat when the car is off, "for voltage", it sould read like 12 volts , close to 12 volts,,if it dones't then your bat is dead, or is bad
then turn your car on, and do it again it should read like 14 to 15 volts some where around there, if it doesn't read that then your alt is broken
but then again i don't know how 80's are wired, but no matter how they are wired the test i just told you will work, no mater, what,, i am just saying maybe its a fuse link, only way to know is to put it on a lift and look for it,, sigh,, , do this test, and it will tell you whats wrong