I don't believe the W111 had the bio-wiring issue anyway. Have you gone though the ignition? A bad coil wire can act just like a rev limiter.
Not W111 chassis, but a M111 engine in a W124 chassis.
W124 E220 (124.022) all come with biodegradable harnesses and wires, including ISA.
Basically, it is a HFM M111 engine in a W124 chassis, like its twins, W202 C220.
Date of production of the Main engine harness can be identified by examining the AIRFLOW socket.
It is stamped on the round socket. Check it out with your good eyes.
Besides, cable harnesses for power regulator/generator and starter are also bio stuffs.
Trigger cables for your 2 pack coils are also bio stuffs....
Given your condition, your priority check would be the ISA.
Its part no is 000 141 92 25 and updated to 001 141 02 25 while OEM VDO/SIEMENS no is 408 227 111 003Z (final ver.).
Make sure yours is good and not affected by the bio issues.
The same ISA is installed in other chassis as well so you might visit your nearby junkyard for a good search, hopefully finding a good one.
Regarding the cutting of engine RPM, it is triggered by a safe-protection mechanism of your HFM engine ECU. Thus your car currently is in "limp" home mode.
Exact causes for that might be many.
Common causes would be a power leak/jump because of the bad wire insulation, and the other would be a bad plug seal on the cam cover. Remove the spark plug wire cables and check. Normally after a good clean or installing the seal kit, and together with erasing the trouble codes, you could bring your engine back to normal.
"BIODEGRADABLE CABLES" in that particular years of production will self-destroy....even you have a mint W124 E220.