here's a list of what to do to fix (assuming a non-oil burning motor with idle speed and distributor timing set to spec):
1. buy a good fuel additive like techron and run it thru a half tank of gas.
2. is the check engine light coming on at all? if so run the codes, your o2 sensor might be bad. to check codes, ground out the green diagnostic connector under the hood, on the passenger side of the firewall, then start the car. the check engine light will blink fault codes (if any).
3. if no codes, then the 02 and other sensors are working as they should (or close enough), and it's probably time to replace the catalytic convertor.
rob