Could it be a the catyltic converter

My 1994 ford escort LX with a 1.9L engine. The car has we believe it to have 180000 miles on it. But we know that engine has been replaced in the car at about 130000 miles. The car hard to start up and when do get to start up it very jerky and idles rough. THe car will get up to aleast 60-65 haven't had the need go faster but it takes a long time to reach that speed and when I idle at stop light or something it usually the idle starts to drop and sometimes the engine will stop complete so I have to place in nutral and restart it to get going. What could be going on we have check pressure on fuel line that was ok, we changes fuel filter and gave it an oil change. THe throttle seems to be ok. Just to figure what is causing this also changed the air filter. What could be wrong?