1993 Saturn SL1 with 178,000 miles. I recently bought this car, and was driving god for some time. Later, wheel bearings got shot and I drove the car for a week or two, with the clunking noise. During this time, when I used to start the car in the morning it used to smoke a lot, smelled like burning diesel, no power, won't go beyond 10-15 mph. At this point if the car was in park, I used to floor the gas few times, RMP won't raise, after few times car gets back to normal. If the car was in motin(10-15 mph) I used to let the gas and step on it slowly(as if driving very smoothly), it used to come back to life. I took it to mechanic to replace the front driver's wheel bearing's and to take a look at this problem. Replaced new belt, tensioner and a used computer. Problem was till there when I test drove at the mechanic's. But later on, the problem wasn.t there for 2 weeks. Again, the problem is back. I also noticed at highway's the RPM stays at 3000 for long time and drops to 2500(when it's in top gear) for few seconds and again back to 3000. Also the temp gauge stays between 1/4 and 1/2 for long time but mostly just below 1/2. Sometimes, it's at 1300 RPM during idle(park or stop sign). Mechanic says sensors needs to be replaced. But don't know which one? After the computer change, Gas mileage is poor. Today checked the engine codes... It was showing 32,33 and 45. Changed o2 sensor and MAP sensor. Now the codes 33 and 45 come up. Changing O2 sensor made some difference. Any advice appreciated. Thanks.

Optional Information:
1993 Saturn SL1

Already Tried:
Trips to dealer and parts store.