This is all the crap that coming out when I pull out the Y pipe. The small cat was completely clogged. I got another used with low mileage and install including the exhaust manifold with the two cat. From the way you describe that fell coolant inside the exhaust manifold. I can think that maybe the coolant caused the material that is inside the cat broke into pieces and your cat is clogged. It is for this Rason that you have the problem with your rpm.