Need help!!! Voltage fluctuation w/ new alternator?

1996 Isuzu Trooper, two alternators, two starters since first bad alternator in 08' as well as constant up+down voltage under load. Add the squeal that occurs most of the time and I'm about done!! Please help! Need a new diagnostic or test! Ohh and the belt tensions and condition are all correct. As well as the battery terminal connects....????????