I would have to agree with Chris, depending on what actually is happening. If, when you refer to "a/c", what in fact is happening is that the compressor outside is turning on and off rapidly, it probably is not the thermostat. Most likely your system has lost freon. There is a safety check in the unit that will turn off the compressor when freon levels are low, so that the compressor does not burn up compressing air. You might be right on the threshold of that level, causing the compressor to be very intermittent.
I mention this because the compressor unit can stop independently of the inside blower unit (part that blows air inside the house), and sometimes people can think their air is running, when in fact only the inside unit is, and the compressor is not. Of course, in this scenario the air being circulated inside the home would not be cold.
If what is actually happening is the intermittent shutdown of the blower inside, the possibility of the problem being the thermostat is increased. However, it could also be a broken or loose wire between the thermostat and inside unit. If you have a basic model of thermostat, you may want to consider upgrading to a programmable unit. They can be more efficient, and install just like the original.
If, however, the problem persists, I would call in a professional before something really expensive breaks.