low voltage - engine missing at 65 mph

[lcregan]

93 Transport 3.8 - When lights and air conditioning are on the voltage meter (on dash) drops to 13. The engine seems to be lugging a bit below 65. When going up hill at 65 the engine will miss and the Service Engine Soon light goes on. When the air is turned off the volts will go up to 14 and the problem seem to go away.

[LMP]

Something very similar happened to mine. One diode (one is enough) in the six diode bridge pack had failed. This is part of the alternator and the whole bridge needs be replaced. A failed regulator (also part of the alternator) might also do that kind of behaviour but the way you tell it points to alternator not able to deliver heavy current, but able to deliver proper voltage at light loads which a failed regulator would not allow, generally: when failed, you loose it all.
THe original DELCO bridge was a P.O.S. and replacement with a BOSCH bridge is better. This part is about $25. YOu need a heavy soldering gun to unsolder and resolder the new bridge to the alternator windings. Individual diodes can be tested once the bridge is disconnected to be sure you have the culprit...but the delco bridge almost decomposes in indescribable chunks.
A year after I changed the bridge, the regulator also failed...another $25. did it right. ALl of this is much less anyway than a new alternator...Of course, may be you can have one at the scrapyard, just hope it is better than yours. BTW, if bearings prove somewhat rough, may be you should consider it gone, anyway.
Notice there are 2 small holes in the alternator back cover : insert a straightened paperclip to hold the brushes when reassembling the alternator.