Use;
? _rand <= 1 :goto ......
? _rand <= 2 :goto ......
? _rand <= 3 :goto ......
? _rand <= 4 :goto ......
? _rand <= 5 :goto ......
etc
This way, if _rand = 0.26 it finds the first line and as its less than 1 it goes to the first place. If _rand is 1.96 it goes to the second place, as its less than 2. And so on. You dont need to state an upper limit, as the script will stop checking _rand after it finds the first condition that is satisfied.
If your mission is going to be played online, you'll need to stop the clients from generating their own value of _rand, as they will all come up with different values.