Take a look at the transmission fluid and see if it is gritty and/or dark colored. If so, you may have some clutches burned out in the transmission. Also I beleive your transmission has the 1, D and O shift pattern. If so, do you normally drive around in D or O shift position? If it happens in the O position (for overdrive) try shifting it back to the D position and see if that makes any difference. If it does then your overdrive may be gone. Heat is a killer on automatic transmissions and so is low tranny fluid levels. These are two things you need to keep a close eye on with automatic transmissions.