I received purchased an Exc. condition ETRSi off craiglist. Today I went out to test it and found that it has a strange shutter problem. I found that the shutter does not fire from time to time with the combination of cable release and mirror lock up. The shutter fired with cable release and mirror down, or with shutter button in the front and mirror up. The only combo that has problem is cable release and mirror lock up and the shutter won't fire for about 50% of time. In these problematic cases, returning the mirror lock up level to the "off" position triggers the shutter. I tried with film back on and off and that does not make much difference. I am wondering if anyone here experienced similar problems in the past and could give me some tips on what is wrong with this body. Many thanks!