What was found
AT FL390 WITH OAT -58C, BEGAN TO RECEIVE INTERMITTENT PACK L EICAS MESSAGE WITH CORRESPONDING CABIN TEMPERATURE DROP. COCKPIT TEMPERATURE WAS SET TO 26C AS COCKPIT TEMPERATURE DROPPED TO 10C. DECENDED TO FL350 AND RAN CABIN TEMP COLD QRH CHECKLIST THAT DROVE US TO L-PACK OFF IN ORDER TO SUCCESSFULLY HEAT THE COCKPIT. FIM 21-61-00-2-0 FAULT FOUND TO BE INTERMITTENT. SWITCH CYCLED FOR PRECAUTIONARY REF FIM 21-61-00-2-0. AMM: 36-11-16-4-4, AMM: 36-10-00-5, INSTALLED LT ENG FAMV IAW MM 36-11-16-4-4. OPS CHECKED NORMAL IAW MM 36-10-00-5. ACP ENG SIM RUN STEP 4. ACFT OK FOR SERVICE.