/cast random spell on gcd
/cast second random spell on gcd

hasn't worked since PreBC days from memory.

The macro will try and cast the first spell but if it is on cooldown it will not proceed to the next step. The exception is if you have a spell which is off GCD like PoM or most cooldown type spells or the occasional spell like wind shear for shamans.

One way you can get around this if you are using a program like isboxer is to bind two macros or spells to the same keypress. The unfortunate thing here is taht if there is a third spell you are spamming then occasionally that third spell will occur before your Lavaburst and may consume the beff from unleash elements.

Either way, you have your castsequence right now so that should be fine :P