Last week all my idiot lights came on while driving my 300ZX. They go out when I am at a full stop.

I had the charging system checked at a Shuck's(Part's America) store. The instrument they used measured a 12.42 charge, but it stated that the "diode ripple" was not detected.

I am still able to start my car. Do I need an alternator, or is there another electrical problem?

No ripple generally means the alternator is not charging. Three easy reasons for this is a broken regulator (which is in the alternator) and a broken wire to the rotor in the alternator or all diodes are shorted.

If 12.42 volts was the battery voltage stopped then battery is OK. If it's electrical system voltage when the car is on then you have a bad regulator in the alternator.

There is not enough information. They should have been able to supply you with a piece of paper with a lot more info on it, particularly voltage of electrical system when running and charging current with all accessories on.

I had two unusual alternator problems in my life time. 1) Car just died. No idiot light. It was a broken wire to rotor in alternator. 2) I don't remember the symptom, but it was a bad bulkhead connection. I think this was an idiot light, but the alternator wasn't bad.

Suspect alternator but not enough information given.

Yes, you need to replace the alternator or have it rebuilt.