Random Draw Question

I need help with a random draw formula. Here is the situation

There are 8 teams with 5 squads each for a total of 40 squads. Each squad is numbered 1 thru 40. Each month a random draw is held to match two squads to compete against each other. However, you do not want anyone on squads 1 - 5 competing against anyone else from squads 1-5. IE: team mates, and you would never want two squads to duplicate any of the previous matches in following months. This draw would occur once a month for at least 6 months.

1 =RAND() =RANK(B1,$B$1:$B$40)
this formula works for random draws, but it does not exclude team mate against team mate and could duplicate a pairing in following months.

suggestions???
Thanks
Chad Kellner