You could try changing the fluid and adjusting your clutch but sounds likes its on the way out. You can find a good used one for $400 or under so I'd just find one and swap it. 2 buddies with a lift can do a trans in an afternoon real easy.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.