My central air is blowing cold air then warm air then cold air again WHY?

that cycle continues periodically through out the week usually in the late afternoons to early evenings and usually blow warm air for about 45 minutes then back to cold air. By that time it has already heated up the house dramatically. We can't seem to get the problem fixed because the contractors that have come to look at it has not been able to find a problem with the system. One contractor even came out when the air was warm and changed a little thing that looked like a mini Oil filter on the condenser and said it was the wrong size and that still didn't help. Also, its running continuously with out shutting of Day or Night. I just hope somebody can help. Thank You