sounds like a pcs, id take it to a shop and have it scanned to make sure. if the cel is not on yet, the auto zone free scanner probably wont pick it up, thats why i say take it to a trans shop or shop with good scanners.

my car did the same thing, in 3 rd gear, it would chug and the tach would jump anywhere from 200 to 500 rpm, back and forth till you let off or gave it more gas, a month later i lost 3 rd gear, and a month after that i got a used trans put in.