Sounds to me like your either pulling aggro from your voidwalker or your taking on too many mobs. when fighting a mob always open with curse of agony then corruption and wait for your void to get a couple taunts off before you use immolate, since that pulls quite a bit of aggro.
also i would recommend going 5 imp corruption then the rest into demonology since your pretty much relying on your pet to survive
try something like this
Talent Calculator - World of Warcraft for a level 26 lock.
hope it helps