Pontius, on the 737 NG, in normal cruise you will have FMC SPD.....VNAV PTH
Then at top of descent, after setting a lower MCP altitude it will switch to RETARD....VNAV PTH then ARM (white)....VNAV PTH when thrust levers reach the fully closed.
In this mode the path only is maintained regardless of the speed and if the speed exceeds more than 10 kt the target FMC you will have a "DRAG REQUIRED" message. If the speed drops by 15kt or less the Autothrottle will be "back into life" as you said and you will have on your FMA : FMC SPD ....VNAV PTH till the aircraft reaches the target FMC then RETARD & ARM again.
That is for VNAV PTH
If your aircraft is equipped with SPEED INTERVENTION (Optional) and you push the button SPD INT under VNAV PTH your FMA pitch mode will change into VNAV SPD, and the airplane will descent on IDLE power at the current or selected speed (speed window will be opened on the MCP) Here similar to LVL CHG until you reach the altitude selected on the MCP or the altitude programmed in the VNAV (in case you have an altitude restriction) whichever is higher. If you have altitude restriction in the FMC which is higher than the one set in the MCP the FMA pitch mode witch automatically to VNAV PTH again. If you want to continue descent you can delete the altitude restriction from the FMC legs page or just press the button ALT INTERVENTION (Optional also)
I guess the 767 is slighlty different but the basics are the same?