What was found
AIRCRAFT WAS GROUNDED: APU WAS RUNNING AND THE LIGHT EXTINGUISHED. AFTER 15 MINUTES APU SHUTDOWN AND THE R IGHT SOURCE OFF LIGHT CAME BACK ON. AFTER QRH PERFORMED DUE TO SOURCE OFF LIGHT, APU WAS RUNNING AND THE LIGHT EXTINGUISHED. AFTER 15 MINUTES APU SHUTDOWN AND THE RIGHT SOURCE OFF LIGHT CAME BACK ON. QRH PERFORMED AND DIVERTED TO THE NEAREST SUITABLE (NAS) FOLLOWING THE CHECKLIST. EMERGENCY DECLARED AND AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED APU OIL COOLER IAW AMM 49-91-41.