My '97 Civic will start fine then stall out after 15-20 minutes. While idling the speedomete reads 30kph, then jumps to 80 when about to stall. Some lights on the dash turn on and the stereo shuts off just before this happens. Once it stalls, it won't start back up for about 30 minutes then it all happens all over again. I've changed the battery, distributor cap and rotor, tested the alternator and even checked the oil. What could be the problem??

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

Here's what the ICM (prime culprit) looks like and costs:

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Ignition-Module-Control-Unit-BWD_18240176-P_194_R%7CGRPTUNEAMS_668202614___

Here's what the coil looks like and costs:

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Ignition-Coil-BWD_19100046-P_190_R%7CGRPTUNEAMS_667383153___

These components are breaking down under load (heat)--very common. I recommend replacing them every 120,000 miles or 10 years, whichever comes first.