Key will not always work when starting on 1999 Mitsubishi Eclipse

Have replaced the ignition lock but has not eliminated the problem. Sometimes it will start and sometimes nothing happens.

Asked by for the 1999 Mitsubishi Eclipse
Does the key turn OK? If not, you need to determine why. If it's OK and it won't start, I suspect that the starter is failing. Check that you have battery voltage at the big starter cable, and battery voltage at the smaller START wire at the starter when your assistant turns the key to START. If so, and the starter doesn't run, then it's faulty.
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Manual or automatic?
1 more answer
If it's manual transmission: your clutch safety switch may have worn out, or fallen out.

Battery needs replacement,
Loose battery cable connection, and/or grounding issue.