Transmission question
 

ok so i have a 200 buick Regal and when i drive it it seels that it goes in and out of gear when i am a constent speed does this mean my trany is going? or fluid might be low? i am going to check fluid but just wondering if anyone has any ideas to help like it seems to go in and out of over drive not normal gears it shifts good just when constint speed goes in and out of over drive please any sugestions will be appreciated thanks