Issues starting 1999 zx2

I have an issue with starting my 1999 zx2. When I turn the key, I hear one click, which I assume is the starter engaging, but no crank. If I continue to hold down the key for a few seconds the engine then cranks and starts up. It doesn't always do it, just most of the time. I don't know what could cause a delayed start like that. I've put two new batteries in this past year. Alternator tested good. Any ideas? Could the starter be going bad?

I have a 2002 zx2 with the same problem a while back, but my problem was that the ecu (emission control ) module was going out, its located under the cup holder which was a good spot for spillage to happen which got into the harness and ever since that the car would not start. and when i got it to start, it only ran for a few seconds. but from what your saying it sounds like it could be anything. i have had problems in the past with the alternator being crap and the car still running on the battery till it could not run anymore. fuel filter problems etc.