2002 Mit. Galant Low Voltage (p1500)

My 2002 Mitsubishi Galant (4 Cylinders G 2.4L MFI SOHC (http://www.autozone.com/autozone/ymme/selector.jsp?asText=true&which=overlay&ymme=32738802) w/ 97,000 miles) has been intermittently experiencing low voltage across the battery. Every morning, (after the car has been sitting all night) the voltage across the battery is 14+. (Alternator is charging the battery) As I drive to work (and the engine warms up) the voltage gradually drops 13.9... 13.8.... 13.7.... When I start the car after work (after sitting in the sun and 90+ degree heat) the voltage across the battery is less than 12. (Alternator not charging the battery) It seems like the alternator will not charge if the car is warm (even just from a hot day). I have run the code scanner. I am getting a p1500. I have replaced the alternator and battery recently. I think the battery is good, but the alternator was cheap. I'm thinking I should replace the alternator with an OEM. Any other thoughts out there?? (especially regarding the p1500).

Thank you!!

Another side note... the charging system warning light does NOT come on while the car is running... maybe a short somewhere????