TXV valve replaced, reversing valve replaced, lines flushed and a/c still isn't working! Help!!

Our central air stopped working about 6 weeks ago. We have 6-7 year old Tappan unit. We were getting air through the vents inside but the air was not cool and the compressor outside wasn't running. We had the installing HVAC company come out to take a look at the unit when it stopped working. The repair said the TXV valve needed replaced. TXV was replaced and a/c worked for about two weeks. Unit stopped working again and repair man came back out and said the lines needed flushed because there was likely a clog. Lines were flushed and unit worked for about 2 days. Repair man came out for the third time and the said that the problem must be the reversing valve. The valve was just replaced a day and half ago and today I've come home to discover that my a/c is not working AGAIN. The thermostat was set to 73 on day when the high temperature was 78 but the temperature in my home was 85 degrees. Like all the times before, the air is coming through the indoor vents but it's not cool and the compressor is not running outside. We check the breaker each time and that is not the issue. I've called the repair man again and left a voicemail but I'm concerned that they are just jerking me around and at this rate the repairs are going to bankrupt me. The unit is still under warranty, should we approach replacing the whole system? Are these repairs common?
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