until [ 1 -eq 2 ]
do
....
let "sleepval = $RANDOM % 271 + 30"
sleep $sleepval
done
No guarantees, this was written on my phone. The principle should be clear, though. In ...'s place you put what you want to run, then get a random value from 0 to 270, then add 30. This is the amount of time in seconds to sleep.
There is quite likely a better way, but I think this should work.