Hi there, the ECI (Earth Continuity Indicator) is designed to flash if the circuit detects an earthing fault either in the ring the amp is plugged in to, in its own power cord, or internally...least that's how I got it explained to me by Trace when I bought my brand new 1812 stack with AH400 bi-amp head back in 1984.... gives away age...whilst they can develop faults, it may be worth sticking a Martindale tester in the wall socket just in case, double checking your power cord and if all that checks out OK gettng a teck to have a quick look.
old man in the far north....
Andy