All US 380s had elecronically controlled idle valves. None had thermostatic valves. Some early European 380s did have manual adjustments with thermostatic warmup air valves.
The valve is not being controlled when the motor runs that high with that system. There is a separate controller for idle on 380s and it is known for cold solder joints on the PC board. Over the years we have fixed 75% of the bad controllers with your symtoms by just resoldering the PC board.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician