I had the same problem on my 2005 Ford F250 6.0 PSD. Under a heavy load or heavy acceleration, I would smell coolant and could see where it was blowing out the overflow bottle. It turned out that this was caused by the heads "bouncing" and letting compression into the coolant system. On your Cat, it is most likely an head gasket problem causing the overpressure in the system. Have you tested the actual pressure in the system with a radiator pressure guage? If not, that would be the first place to start. It just replaces the radiator cap and measures the pressure. There is also a test strip to detect combustion gasses in the coolant, and the definative test will be a compression test.