I have seen this problem many times and usually it is a slipping belt. The load from the alternator is very heavy at first, and the alternator will load the belt until it slips, causing the voltage to be low. After the initial starting depletion is taken care of, the load is less, and the alternator speeds up and the voltage rises to normal. Try replacing the belt and/or retensioning it! The alternator you installed is probably identical to the one you took out, except rebuilt using an OEM frame and rotor, so it's probably fine.
'82 300D/4.3L V6