Honda engine idle goes up (2000 RPM) & down (1200 RPM) when warm (6 cyl 98 accord)

. Clean or replace the Idle Air Control Valve (IACV):

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-7.html#post1054149

. Clean the throttle plate and throttle body. Afterwards, clean the air box and replace the air filter.

. Replace the PCV Valve.

If the problem persists, focus on the Ignition Control Module--it may need replacing. These links will provide the necessary background, perspective, and how-to:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post260436

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

Check fast idle valve sometimes they stick open