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.