What was found
AT VI INTERMITENT LEFT GEN OFF LIGHT ILLUM. LEFT AC BUSS FAILED, RIGHT BUS DID NOT PICK UP LEFT BUS. SHORLTY THERE AFTER THE R AC BUS FAILED. MULTIPLE WARNINGS DISPLAYED. RESET R GEN, R BUS POWER BACK ON. FIRED APU L BUS POWERED. SHORTLY AFTER L BUS POWER WAS INTERMITTENT. RETURNED TO GDL AIRPORT. PERFORMED BPCU BITE TEST AND SHOWED AUX CONTACTS/BPCU/GCU MES. SWAPPED APU GCU TO LEFT GCU. RUN ENGINES AND ATTEMPTED TO DUPLICATE PROBLEM. SYSTEM OPS CK NORMAL. DEFERRED APU PER MEL.