By the diagnosis given, there is not much to go on. I would have the distributor checked since they have the tendency to unallign, especially if it was not replaced right. As for 900 dollars that is ridiculous. I am an ASE master tech and I would reccommend buying an o2 sensor 40-60 bucks and a universal cat about 50-75 bucks. That is, if price is an object. I usually do that job for 300 bucks. They are both fairly easy to replace.