hi
usually wrong coolant or mixing different types of coolant
you dont say what model you have, but only models with water/air intercoolers are the supercharged models, every other turbo model uses air/air intercoolers, also intercooler would not cause dirt in the system as there plastic and aluminium
if its oily residues in the water, then best do a sniff test and get the auto oil cooler checked, as on some model the gearbox oil cooler is built into the radiator
but flush it with lots of water and refill with correct coolant, which on most Jaguars is OAT, should be a red/orange colour
cheers
Joe