Why is my Mustang overheating and the mechanic can't find the problem?

Hello Visitor in LV! sorry to hear about your misfortune. I would have a combustion test performed on your mustang asap. The test is performed by way of your radiator or radiator reservoir. Combustion leak detection fluid is put into the tester which has a squeeze type bulb on the top or similar. The test will determine if you have a leak, (no matter how small), in your head gasket. The fluid will turn color, usually yellow, of a leak is detected. It only takes a couple of minutes. The test is very accurate and answers a lot of questions particularly due to the work you have already had done. Expect to pay $80 or less but certainly no more. My shop would do the test for $40-50. Also, check your oil to be certain there is no coolant mixture there, (a light brown or very runny), as well as the coolant/antifreeze appearance to ensure no oil has infiltrated the cooling system. Certainly the shop you visited has already covered those last two. Lastly, a cooling system flush will flush out or highlight any restriction in the system that may be present. Good luck. please post back with your findings, questions, and certainly any fixes. thanks