I would go with what TraXel stated and look at a mixture of INT and Spi over +spellpower / healing (But do not neglect it altogether).
What you stated was that you go oom 40 secs into a fight and that indicates a problem with all three (+SP, Int, Spi).
You'll need to elaborate a bit more though. Are you referring to HC's (Trash or Bosses, Raid trash, Raid Bosses)?
Play style and spell rotation also play a key factor (I know it does for me) and there I have limited knowledge when it comes to priest heal rotations.
Personally I have opted for more Spi to try boost my mana regen.
So for ME it is +Spi then +Int and then +SP.
I have noticed with our raids (limited to 10 mans thus far) that our priests are struggling more with their mana pools than their ability to heal. In other words they most likely have the +spellpower but do not have enough +Int and +Spi.
Here is a link I found on the priest forums. It is a bit dated but wade through it and you might find something useful:
http://forums.wow-europe.com/thread.html?topicId=125452547&sid=1
You could also use the armoury to compare your gear to others on your server and / or use a site like
www.warcrafter.net to get a sandbox breakdown of your gear and what effects adding other gems / enchants will have on your gear.
I have only started with the whole theorycrafting aspect but will post a bit more a bit later as I research a bit deeper into priest healing.