Sure sounds like a Vacuum Leak, sorry could not help myself.
When you say drop, drop to what RPM?
But in all seriousness. when cold after initial warm up idle. Leave the car on and remove the oil filler cap. If she dies, you have a major vacuum leak. If not then +1 on verifying AFR with a LM1 or LM2
Not likely CHTS but check the OHMs, you never know.
