I think its interesting.
And time consuming. For every spell or SLA that's cast, that's a major distraction from the game at hand having to consult the table, roll, consult, and maybe roll, consult some more.
I think the random spells that happen should be limited to maybe 1 higher than the highest level the PCs can cast. A group of 3rd level characters hit by a Prismatic Spray are all pretty much dead; as would be the unlucky group of 9th level characters that have a Gate pop open in their campsite from the wizard trying to cast Alarm!
I'd suggest doing maybe 5 rolls ahead of time, writing down the results and then just applying them as soon as the surge is triggered. That would save you from halting the game just to consult some tables and would make sure that you don't random destroy or cripple the party.
Behind the scenes, it looks to be a decent tool, taken with the comments I've made. I'm sure you could tweak it here and there, but as is, it looks ok.
