i have had to change three cats on focus 1.8 tdci due to it breaking up inside,over time it vibrates and eventualy you are left with what looks like a tennis ball size chunk ! this then blocks the exhaust causing the lack of power. it can somtimes vibrate free again if the car goes over a bump. the early signs are a vibration rattle sound coming from under the car on tickover. its worth checking out by removing the cat,it may not be the fault you have ,but the first one we came across at the dealers took some time to work out.