Cold start RPM

[Benz280]

Trying to identify :
Originally during a cold start RPM would start at 1300 / 1400 RPM . Shortly after ( Warming up ) RPM would drop to normal 800 range. ( This is correct start )
Now, cold start is around 500 then moves up to 800 range when warm.
Replaced thermostat in WRD , no change.
Any thoughts as to what could contribute ?   
   

[ja17]

Make sure that the little air filter on the WRD is not clogged.

[Benz Dr.]

Stuck slide valve under the thermostat.

[Benz280]

Thank you JA17 & Dr. Benz
As for air filter - Working fine , not clogged at all .
WRD pulls strong air when cold , as engine warms valve closes , when warm ( 3 Min )  no air being pulled .