My daughter's 1998 V6 Honda Accord has a problem where the idle oscillates about every 3 seconds between 800 and 1300 rpm when it is warmed up. No idea what is going on. Someone told me that it could be the fuel filter (which to my knowledge has never been changed) but I can't figure out where the fuel filter is. Any idea?

Passenger side of firewall. Here's how to do it:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-honda-civics-46563-2.html#post224689

If changing the fuel filter doesn't solve the problem, test/replace the igniter inside the distributor. Autozone will test igniters and coils for free. In many situations, but not all, igniters cause erratic rpm:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-honda-civics-46563-3.html#post260436