I'm going to have to agree that your flow rate is notably low. If you can get it up to 3.4 l/min or better that should fix the problem mostly.
You are running MB and ram blocks which I believe are big points of flow restriction.
Someone else mentioned the mixed metals of the Zalman radiator tower. I almost bought one when I was ready to pull the trigger on water cooling and am glad I didn't. I wasn't aware of the risk of galvanic corrosion, but I did know it wouldn't meet my thermal dissipation needs.
If you do the rigbuilder thing it'll help other folks evaluate your setup.
- Option 1: One big loop that is reworked.
Ditch the Zalman tower and Zalman CPU block. Replace the tower with a radiator that is 2x120 or larger. Pick a CPU block that you like and buy that. Danger Den, EK, or Swiftech will all be good picks. Add at least one high output pump. The Swiftech dual-top might be ideal for your application -- Which is you need a LOT of pump to push water about.
- Option 2: Multiple loops (not sure how good this solution really is, but I'll brainstorm it anyway...)
If you are running the Zalman CPU block you could just run that and the radiator tower by itself. That's probably all the heat that thing can dump really. You could then put everything else in a second loop with an extra pump. So your GPUs, Ram, and two MB blocks will be in the other loop.
-Option 3: two loops reworked.
Again, ditch the Zalman tower and CPU block and replace it with a 3x120 radiator or larger. Buy a dual pump system and the primary loop would be the CPU and 2xGPU blocks. The secondary loop would be your 2xMB and RAM blocks. You probably won't need much radiator for those parts. I also don't think flow rate is as critical as it is for your primary loop. As a guestimate, a 2x120 will work just fine for the secondary loop. You can probably also reuse your aquastream pump with no issues there.
Option 1 might be the simplest to implement overall since it doesn't really change the structure of your loop radically. It does introduce mechanical problems, but the bottom line is that you are going to need a lot of oomph to get water through those smaller blocks. This is likely your best route.
Option 3 separates the system into more manageable chunks, but ups the complexity of the system. If you are comfortable taking on that complexity and the necessary rework of your hosing, do this.
[ Considered a suggestion to undo the use of water on the motherboard and RAM, but changed his mind... ]