I have seen a lot of posts on this forum on this and other Honda Forums regarding bad ildes when the car is at temp usually on vehicles with 100k on the odometer. The car starts fine and idles great when cold but once it's a temp it starts to ilde erratically and sometimes kills when you lug in heavy traffic and kicks out codes like 300, 301...306. The car runs fine at higher rpms and generally has no other issues.

I tried everything: tune up stuff, coolant system bleeding, cleaning out the intake manifold, adjusting the valves---everything!!!

Finally at long last, I solved it. I installed a 20 dollar O2 sensor from Rock Auto and it idles like new!!!!! The clue was open vs closed loop. A cold vehicle runs on open loop but when hot it switches over to closed. There isnt too much room for error at idle (its like a pilot light on a stove) it barely keeps the flame alive so even a little error from an aged but not necessarily defective sensor will make a huge difference. Plus Honda V/6s, at least on my 99, have only one aft O2 sensor and no mass air flow sensor thus greatly increasing the importance of that sensor's input in the big picture.

By the way the twenty dollar O2 sensor from Rock Auto works just fine dont waste your money on Densos or NGK they are all made at he same 3 or 4 factories in China. The exhaust pipe doesnt care how pretty the box it came in was.