How did it get overfilled? are you checking it correctly (hot, in neutral, engine idling, on level ground)?

remove the excess fluid, or perhaps change it and clean/replace hte filter and see if that helps.

As to overhauling it, it is not something for the first time amateur to try and do. Trans rebuilding is a specialty. If you can r&r the trans you can do an exchange at most any trannie shop, and they will still guarantee the new trans for a year/12k miles. A used trannie is a possibility too, try the boneyards for that.

chris