I guess as the others in your thread have asked, what exactly is the problem? Is the truck not cranking? You say the alternator is good - does this mean you took it out and had it tested? Was the voltage fluctuation happening before you had the alternator tested?
i saw a post you comented on the other day and thought you might be able to help me. i have a 79 gmc k25 sierra classic, and when the key is off i have power to the clip and field wire on my alternator, and no power when the key is on. i have a new battery, alternater tested good, and i installed a new ignition switch.i checked all my battery and starter grounds and they seem fine. about two months ago i noticed on my voltage gauge it would drop to 8 volts for a few minutes and then jump up to around 16 or 18 volts and all my lights would get brighter, and my heater fan would speed up significantly. can you help me?