Ok, so a super long lurker here. I guess that's due to not having any issues with my Honda so far.

Backup is as follows. It's a 2004 LX 2.4l, 5MT with about 102k on the odo. Last weekend I changed out the secondary O2 sensor (OEM) due to a reoccurring MIL that came up with a P121 (going from memory here) error. Also, changed out the drive belt and removed the throttle body completely for decarbonization. I replaced the TB gasket and drove it around the block and all seemed well. Tonight I drove for an extended period (about 20 miles) and the idle was high, started about 2k and settled about 1.4k cold.

After warming up and shifting to neutral from a higher gear, rpm's would dip to about 800 then jump back to to 1.4 to 1.5k and then over about 7 seconds slowly drop to about 900 rpm. Before removing the TB I consistantly idled at ~800 rpms. What's going on here? Is it a vacuum leak? Misaligned TB?

I performed the idle learn procedure tonight after disconnecting the negative terminal for 10 minutes, but nothing seems to have changed.

Please advise!

The IAC valve is probably stuck open. This is a common issue when the tb gets cleaned, the cleaner will make the coating on the valve get sticky and the valve will stick open.

The jumping up and down is the PCM cutting fuel and timing to keep the idle from being so high. It should eventually throw the MIL and set a code for either IAC or idle too high.

You can try to clean the valve again and hope it frees up, or what most times happens, replace it.