The old days were full of multiple repairs on the same unit for the same problem as well, so that's not new. ( I know, its always worse when its yours.) Some ideas to help:
1.Demonstrate and duplicate the problem to the store manager or clerk that writes up the repair ticket, They might word the symptoms differently which might make the tech look further You could even use your computer to record a CD of the audio and send it along this time. It may not be as obvious to them as it is you.
2. Be sure they write on the repair that the unit needs to warm up x minutes before the problem appears.
3.Suggest (insist nicely) that they send it to a different repair center, namely the Fort Worth one. If your store says they don't wan't to pay again for sending it to different center, (kindly) suggest they contact the District Manager who can assist them with transfering charges.
4.Don't degrade anyone or anything about the process (more flies with honey, etc.) while trying to get this resolved.
5.Lastly, remember the store doesn't want to deal with an unresoved complaint any more than you do.
They want to see you because you're happy and want to spend more money. They are probably just as frustrated as you because you may not be their only customer with an unresolved repair issue at this moment.