MD80 Captain reports ATT and HDG comparator lights illuminating at FL310; the decision was made to divert for repairs. After overweight landing inspection Maintenance discoverd component loosely secured in rack; likely causing the problem.

Narrative

Shortly after crossing into U.S. airspace; instrument comparator lights came on. Displayed on the First Officer side were steady; 'ATT' and 'HDG' lights. On the Captain side were flashing 'ATT' and 'HDG' lights. The course display window on the Captain's side showed rolling numbers. The Captain's PFD displayed only frozen flight director bars with a red 'ATT' message; and his ND was blank except for a red 'HDG' message. A popped 'VHF NAV' circuit breaker in the overhead panel was also discovered. We consulted the QRH; the Dispatcher and Part I. A decision to divert was made; for reasons of weather; nearness and availability of customs service. The aircraft was landed at about 133;000 LBS landing weight; using overweight landing procedures from the QRH and the flight manual. An overweight landing inspection was accomplished prior to the next leg. Maintenance discussions on the ground mentioned a loosely secured navigation box had been discovered. Flight then proceeded to destination without incident.