Car Vibrating

1993 Nissan maxima, 113,000 miles, 6 cylinder automatic. Bought 6 years ago at 95,000 miles. Had since changed Timing belt in 2004, struts, ball joints in 2005, rebuilt injectors in 2006 and axle boots, timing cover seal and oil pan. Gasket, camshaft July of this year. It was woking fine until I took it for an inspection and the mechanic said the ankle boots are cracked and so I had them do the job listed above. A week or so later, I had to fight my steering wheel, when I make a turn because it would not move, I returned the car to them, took them 3 days to tell me that they cannot find anything wrong with the steering wheel, but that there is a leak in the front pipe that connects to the catalytic converter. That if it is not replaced, it will mess up the sensor. I was not too happy with them so I passed. Now, 2 weeks ago, my car started to Vibrate, like when it is ready to stall, I took it to my old mechanic ( he does not do Inspection), for a tune up, thinking it is all I need, the problem did not disappear. I even had my brakes done. I was told one of my cylinders is not working and it will cost me a bundle to have it fixed. My dilemma is, I had already put enough repairs to this car to just give it up, maybe if I will have the problem fixed, I could keep it for a few more years. Or will it keep working even with just the 5 cylinders? I only go 3 miles each way to work and back. Oh, I was also told I am loosing antifreeze, eventhough I do not have a leak in my radiator. Please give me some feedback, thank you.