normal engine deceleration
 

Suppose I'm in neutral and I rev the engine up to, say, 5000 RPM and get off the gas. Should the engine speed return smoothly to 800 RPM?

Mine returns smoothly to about 1500 RPM then sputters down to 800 RPM.

Sounds like yours is working correctly. If you have a PFC, you can adjust the rpm where it "catches" itself. Essentially, when you let off the gas, it completely turns off the injectors for emissions reasons, but at some rpm it needs to turn them back on to keep it from stalling. That's why it hesitates at 1500.

Thanks a lot.