1991 Isuzu Stylus Acceleration Problems

1991 Isuzu Stylus 4 cyl 180000 miles

My 91 Stylus with 180k miles (my first car so I'm pretty clueless). A couple weeks ago I was on the freeway and my car stopped accelerating. It would not go past 45mph even though my foot was on the floor. After a couple miles it jerked than began to accelerate again. A couple days later the problem happened again but this time while driving on the street. I stopped at a red light then my car will not go more than 15mph. A couple blocks later with my foot on the floor, it jerked, went to 20mph and the engine light came on. After another stop light it went back to only 15mph and the check engine light disappeared. So far the only work I've had done on the car is replaced the alternator and get a tune up. My mechanic wasn't able to tell me what's wrong but he did say I have to replace the catalytic converter because it rattles. Will that solve the problem? If not what else should I have checked on the car and what type of mechanic do I go to. (My mechanic was from an electrical/ smog place). Thanks a bunch.