If you were only "told it was tuned" with no receipt then I would assume it wasn't if I were you. Not to mention nobody with a tune is going to put a #30 MAF in there. Or if you did that would be why it is idling crappy. If it really was tuned then the tuner would have calibrated the fuel tables in the computer for whatever MAF was in there.
Need more details but if its an HCI car and not performing well a bad or stock tune is usually the first culprit.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.