Though you may have done this alreay but ive had a few w124 and w210s towed into my shop with similar problems from other shops who have tried countless things (IPs, injectors, filters, ect..) ive found that the fuel hoses relax up on the fire wall and suck air, seems to only affect 606 and 604 engines, and only cold. Just grab the hoses and see if they will spin on the steel pipes on the firewall on the drivers side. If so replace them all from the lines to the fuel thermostat in the head.
Keep us posted
Good luck
Ben D