"and +103 spell power from items."
Blizzard overall gets very little from spellpower. AoE gets 33%. Channeled spells get 20% every second if they're 5 seconds long for 100% over 5 seconds. If they are above 5 seconds, they do not get additional spellpower added, only less added every second.
Making it worse for Blizzard and Hellfire, compared to Fireball (100% added) or Frostbolt (80% added)
Have you tried dropping some spellpower gear pieces and focusing on INT/Stamina where possible?