Have you done any mods to the car that would account for excess power consumption? There should be nothing going on in a 124 that would require more than the stock 80A alternator. Upgrading the alternator without solving the root cause can cause more problems than it fixes.

The stock alternator will vary from 12.5 to 14.2v depending on load, battery condition, RPM and temperature. You may briefly have even lower readings under extreme conditions, like having the electric fan engaged while the headlights, a/c, electric fanand blower are on and you are idling for awhile. But if you're not getting at least 13.7 at road speeds, that suggests either a bad alternator or a drain somewhere in the system. I would suggest that you begin by cleaning the ground connections, cleaning the battery terminals, cleaning the main 12V+ bus connections, and checking the engine ground strap. Finally, remove the VR, and dress the slip rings on the rotor with an abrasive. If that doesn't improve things, try pulling fuses to see if any of the circuits are drawing unusual amounts of power.