Finally started...transmission slipping?

[1966stangkid]

I just got my 66 i6 automatic to start.. some of you may know that i bought it two months ago when she wouldnt start for anyone. the guy i bought it from told me the car hadnt been driving well before he parked it and let it sit for two years. .......now two years and two months later it cost me 60 bucks to put a new batt. in and put in feash gas. anyway she starts right up now and sounds really good. the problem is when i put into any gear and press the gas the moter revs for a little and then she jerks foward and starts moving. but if i take my foot back off the car stops and i have to repeat the process again. does anyone know if this is the transmission or the rear-end slipping. and if you could tell me how to go about fxing it. thanks guys.