2004 Honda Accord EX 2.4L, 294,000+/- miles.Idles rough/shakes. Worse when idling in D. Even more so when AC is on. Once driving, it smooths out. The purge valve on the block has been changed, harmonic balancer changed, engine/motor mounts have been replaced within past year. P0420 code( catalyst efficiency) is active. The mechanic I normally use can't figure it out at the moment. Says it may have to get worse before he can accurately diagnose. Any advice, tips or help is greatly appreciated.