I would check the instruments (again) to check if the pass flow is correct. A lower flow in one pass can make it coke up faster than the others. Also check the burner fuel gas flow, you might be heating up the coil more in this pass than the others.You said this pass has given you problems in the past, have you done infrared imaging of the radiation section coils ? Have you compared the termocouple temps to infrared imaging temps in the same tube section ? If your spalling was not succesful you could have a spalling steam/ velocity steam flow problem in that pass, again check your instrumentation.
Hope this helps