1994 Mercedes Benz 190e 1.8 Sludge in the header tank

I noticed when the coolant light came on and I checked the water levels in the header tank that there was a brown sludge in there. I took the tank off and flushed it out as well as the connecting pipes but it seems to have come back. There is no white residue around the oil filler and the oil seems fine I took it to a garage and they gave it the sniff test to check head gasket but told me the gasket seemed fine. On cold mornings the car is sluggish and judders with no acceleration and sometimes stalls in reverse. I would be very appreciative if someone could help.