What would cause your compressor to only come on sporadically when the central AC turns on and would the whole unit need to be replaced or just the compressor itself?

It could be that the outdoor fan is not turning causing the compressor to overheat and shut down on overload or your outdoor unit may need to be hosed off. You could have a faulty run capacitor. If you do need a new compressor you have to consider the age of your unit, the need to replace the indoor unit, and your financial situation. The cost of a compressor is over 500.00 in my area, then you have the labor, refrigerant recovery, and misc charges. It could bring the total of the job to 1000.00. Complete change out could run between 5000 and 8000 depending on size of equipment and geographical locale.