Mercury - Sable :: RPMs Went Up To 3K And Speed Dropped To 15 Mph

I recently purchased a 2004 Mercury Sable with about 112K miles on it. I am having some troubles with my car and I'm not sure where to start first in repairing it. I recently took my car to a mechanic because accelerating on the highway or going up hills made my car sound engine run very hard and loud, my RPMs went up to 3K and my speed dropped to 15 mph (which is a disaster when you're on the highway and hit a steep grade). My car absolutely cannot do any sort of hill so the mechanic suggested that either he or I replace one of the catalytic converters. He showed me which one he thought was throwing the error code. I replaced that catalytic converter and I thought my problems were solved because I could accelerate without difficulty.

However, a day later I tried to drive it up a hill and got the exact same response as before. RPMS jump up and the speed drops down to 15 mph and I've just about got the pedal to the floor. I have friends who repair cars as a hobby and they disagree with each other where I should go from here. One friend says replace the muffler and resonator (possible blockage that cannot bear any stress under a load...like going up a hill). Another friend says replace the third catalytic converter (one was replaced when we bought the car by the dealership, and I just replaced another one). He says when one catalytic converter goes the others are not far behind.