1991 Chevy Camaro no response from ignition

The problem is; sometimes it cranks, sometime not. It doesn't seem to matter if the the engine is hot or cold. I have put less than 2000mil on it since a total tune up.
When this problem begin, my mechanic replaced the battery cable ends then the battery followed by the starter. Still it does the same thing. When you turn on the ignition the guages and interior light stays on but it won't even click. Its like the ignition is connected to nothing. I am a single woman and though I trust my mechanic.