That is the tell-tale of your oil cooler requiring rebuild or replacement (depending on the route you want to take.
My advise is to do the job in this order:
Conduct your coolant flush (do this before taking things apart so you can use the dab cooler to catch debris in your coolant system before it flows through your new cooler).
-Re-filling with Silicate-Free ELC (preferred), thoroughly flush with Restore, Restore Plus, and then Distilled water.
-Re-filling with Ford Gold (not the best option), thoroughly flush with VC9, then distilled water.
Solve your EGR cooler (based on your build)
-Upgraded unit from Bullet Proof Diesel
-Blocked unit
-EGR deletion
Rebuild/Replace your Oil Cooler.
Clean out your intake manifold and EGR valve.
After everything is back together refill your coolant.
You may want to consider a coolant filter as well.