Transmission issue

Runs like a top, but when I start driving at about twenty miles per hour (shift mark) when it is supposed to shift it feels like it is almost popping into neutral instead of second and it will just sit there and rev. If I put it in park, and shut it off for about a minute or two, it will do the same thing, I put it in drive and it goes until it shifts out of first to basically neutral. Could this be a clogged trans filter? Or am I missing something? I do not want to do it unless it is a for sure.