I read the answers on the site. Anyway to narrow the problem down?

I have a 1999 Honda Accord. It has never had a problem starting. Over the last month the RPM gauge has dropped down to zero while driving it - either on I95 going 60 mph or on back roads. If I step hard on the gas it picks back up and continues driving. A few times I have lost all power and every light on the dashboard has lit up. I put it in park and started it again with no problem and was able to continue driving.

I took it to a mechanic who ran a diagnostic test that came back showing nothing wrong.

Where should I start?

Replace the Ignition Control Module (ICM) and coil:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post265896

This should solve your problem.