freezer tripping circuit?

Westinghouse f/f -17 yrs old perfect cond
Has started tripping power board every ONE AND HALF hrs? -have disc. all sockets and appliances -isolated to just f/f - even moved to another socket or 2 -still same
have cleared items from freezer fan vents
when unplugged f/f nothing trips
HELP

HELP AUSSIE
January 2018