Transmission problem?

In a 98 accord v6 auto; what would cause the transmission to lag or stick between shifts? This problem mainly accurs between 1st and 2nd but has happend between other shifts. The car seems to only have this problem when it has been driven for a while, leading me to believe that heat could be a factor. Im am not really up on accords for tranys for that madder so any help or suggestions would be appreciated.