1993 Z28 (LT1 motor) will not always start. Problem may only occur once
every 1-2 weeks, typically in the morning after a heavy dew.
Car cranks but will not turn over. A caughing sound comes from the exhaust
after key is let go. The Opti-spark unit has recently been replaced.
What else could it be?

 

Next time it happens check to see that you have a good spark & check that all wires are good and tight and in good condition. Optisparks hate water and dampness, but I don't think this is your trouble. Is the engine lite going on?

 

All spark plug wires and boots were replaced when the new Optispark was installed.
The engine light does not come on at time of no start. However, intermittently comes on
while driving (not every time and it usally goes off). Very frustrating.

 

Try to get the car scanned & see what the trouble code is, there may be retained codes, or drive arounf with a scan tool plugged in & see what comes up when the light comes on. Also when it fails to start, check that you have a good spark, and check the fuel pressure with a gage at the fuel rail, just in case the fuel pump fails to come on, you could have an intermittent pump relay.

 

Is there any chance that the newly replaced Optispark unit is the problem?

 

I guess anything is possible, but not likely. Did you have this problem before you changed the Opti? You may want to check all your work, and see that all electrical connections are clean and tight.

The problem with intermittent troubles is that you can only work on them at the time they are happenning. Get the car scanned & check for retained codes, at least you will know what the trouble lioght is about.