Have you changed the Positive Crankcase Ventilation (PCV) valve? It is easy to replace and costs about $5 USD. Check carefully the tube that connects the valve for cracks/air leaks. Check the other vacuum hoses connected the intake manifold.
Also, the engine may just be dirty. Add Marvel Mystery Oil (or automatic transmission fluid) to your gas tank at fill-up at a treat rate of 2 oz. per gallon. Drive the car immediately in stop-and-go traffic for 10 minutes, shut off engine for 10 minutes, then drive on the highway at 70 MPH for 10 minutes. Repeat this cycle multiple times, for at least an hour, and allow the mixture to penetrate at least two days.
When your tank drops to 10 gallons of gas, add two cans (15 oz.) of Berryman B12 Chemtool (or, if unavailable, one ounce acetone for every gallon of gas). Immediately drive the car on the highway for 30-60 minutes, then "floor" the car at wide-open-throttle (WOT) from 50-80 MPH. Repeat the full throttle blasts at least six times, but up to 30 times if the cat. & sensors are very dirty. Be sure the cooling system and transmission are flushed and working properly beforehand. Turn the heater and fan on maximum, with windows open, when performing the WOT blasts. Repeat this cycle by driving the car in stop-and-go traffic, shutting engine off for 10 minutes, then driving on the highway (you can substitute highway speed with driving the car in first gear, 25-35 MPH, at high RPM to develop heat).
The Berryman's (acetone) evaporates out of the gas tank after about 24 hours, so be sure to run the car hard right away. The Marvel Mystery Oil never fully evaporates. The two products – Marvel Mystery Oil & Berryman”s B12 Chemtool -- work together and will never damage any components of the car.
When the problems are resolved and you refill the tank, continue adding Marvel Mystery Oil at a treat rate of 4 oz. for every 10 gallons of gas. It will continue to clean deposits. Every fourth fill-up, add one can of Berryman's and drive at highway speeds.