I'm thinking something with rear band or reverse servo if you have 3rd gear. The front clutch and rear band apply to give rev.The front clutch is also applied in 3rd. The creeping in neutral maybe caused by a lack of clearance in the rear clutch.If you have 3rd I'd wanna pull the pan and check the reverse band&servo. I don't know If I'd condemn the valve body until I looked. If you do decide to change the VB check the rear band application by air checking it.