Volvo: 86 Volvo 760 GLE Turbo tough electrical, volvo 760 gle, windshield wipers

Question
I have an 86 Volvo 760 GLE 4cyl turbo automatic.  There is apparantly a problem with the alternator/voltage regulator as when the engine is running and the positive battery terminal is disconnected and a load is placed on the electrical system such as the headlights on the engine dies.  Replaced alternator/voltage reg with a used one on a spare wreck we have and it now runs with positive terminal disconnected as long as the headlights, ac fan and/or windshield wipers are not run at the same time.  With the battery connected and running highway speed I have erratic engine when running electrical components.  When running Headlights, AC fan and windshield wipers the wipers wont work.  Pulling over, turning off the lights and ac fan for a minute fixes the issue.  Whenever there is a load on the electrical system such as running lights the car runs rough, no acceleration.  I dont know if its the alternator, voltage regulator or an excessive draw by the lights ect...  I dont have the $$$ to go changing parts till I get it right.  Your opinion is appreciated

Answer
Noel,
I am not sure the way you tested the charging system is valid. Once the battery is low and you put a demand on the system it sounds like it fails. You will need the help of a tech to try testing and searching out the faults. Ok put a volt meter on the battery what it the reading? Start the car what is the reading? Turn on lights etc. what is the reading? Is there a part store in your area that can test an alt if you bring it to them?