Thanks to you all for your advice however, the charging mystery on my 1993 Volvo 940 continues. I have replaced the battery with a new Interstate battery, had the alternator rebuilt (Denso 14v 100A), checked and cleaned all wiring terminals including the ground connect and noted the following:
(1) With the engine off, the voltage across the battery terminals is ~12.7v which is normal.
(2) The car starts easily and, with the engine running, the voltage across the battery terminals reads ~14.4v which indicates that the alternator is charging properly.
(3) After driving the car for about 20-25 minutes, while leaving the engine running, the voltage across the battery terminals reads about 12.3v. This indicates that it is not charging.(No lights or fan were on during the measurement.) Charging appears to be intermittent. The belt from the crankshaft pulley to the alternator is properly tensioned. So, I am baffled???

The only thing that I have observed is that when I start the car, sometimes all the dashboard lights come on(which is normal when the key is switched to the ignition on) and sometimes only two lights come on. In either case, the car easily starts.

Any insight will be appreciated.

Many thanks,

OnlyVolvo