You need to seed the randomiser - depending on the seed it will generate different values, otherwise it will keep the same pattern indeed. The command you want is Randomize <int> - replace int with an integer value to set as the seed. But if you specify the same integer every time you will end up with the random numbers you generate keeping the same pattern again... so you need an integer that always changes. The internal system timer is a good start ^_~ The command you want there is Timer(), it returns your internal system time in miliseconds, its forver changing. So, before a=rnd(blah), insert this:
randomize timer()
that should work fine (i hope, otherwise i'll look like a moron XD)