2007 Mustang GT Automatic Transmission Acting Funny

This has probably been happening for some time but I suspect it was fairly gradual and I didn't notice it until recently. Car doesn't always shift properly. It's mostly when I am slowing down and it's attempting to downshift, it tends to go past the normal shipt point and then kind of jerks into the next gear after the hesitation. Sometimes it does the same thing when upshifting, but not as much. I think I can notice it possibly slipping a little when turning corners, especially right. Is this possibly just a low fluid condition? There is no dipstick so I can't check the level without removing the torx portion of the drain plug. Any advice would be appreciated. I am willing to try and top off myself and avoid a large service cost if at all possible, but wanted to run it by you all first. Thanks!