Power in my 307 hdi?

I have a peugeot 307 1.4 HDI 2002 and there is very little acceleration in any of the gears and i can only get approx 75mph max and that is going downhill. Would it be CAT prblem and so is it possible to fix without a garage being involved. If driven hard it has overheated and there is a burnt smell is this an indication of a bigger problem. Mechanic previously had it on a computer and it came up with an air intake valve which he fitted. No difference. Car had been sitting for approx 4/5 months without being driven. I am not a mechanic but will try and tackle if possible.