A good way to improve surecast and make it not suck.

Instead of making it timed, turn it into a self-cast buff with a longer recast time. This buff gives 3 stacks, and each stack will prevent one spell from being interrupted and will vanish whenever you would normally be interrupted. In short, it doesn't vanish unless the spell you're casting would be otherwise interrupted. Still shouldn't allow you to move and cast, but it'll give you options.

There we go, we have a skill which has a use and is now a buff you actually want to keep up at all times. Set the recast on par with big skills (5 mins) and it remains balanced.