Using 4 slots = FPS down on 4?

Hello everyone. I'm having a HUGE problem. I recently purchased 3 more Diablo games in order to multibox - hell yeah baby!
I'm on trail periode atm(which is great!) but I'm having a problem which makes my gamer heart cry.

When I load the program and my characters everything is fine - UNTIL the 4th slot is being loaded.
My computer is:
2500k - 4.2 GHz
16GB ram
GTX 590

I have all my drivers up 2 date and no spyware / virus'.

I have taken 3 screenshots which shows what is going on:
Everything is fine here - playing with 1 slot.
http://img27.imageshack.us/img27/7696/1slot.jpg

Then with 3 - FPS drops down to 90 but thats to be expected when playing with 3 slots.
http://img546.imageshack.us/img546/2877/3slot.jpg

Now.. When I turn on the 4th slot it goes crazy.. A drop from 90-100 fps to 4fps?!
http://img163.imageshack.us/img163/8681/4slot.jpg

What in the name of god is going on here? I would say my computer is a killer.. come on? 16 gb of ram and a freaking GTX 590! I feed my GFX with small children each morning for gods sake! How on earth can people, with an even worse computer play with 4 slots and have no issues at all? It doesn't make sense? I even saw a guy play with a nVidia 460 card using 4 sloth? Whaaaaat?

Anyone got a solution to this? I would LOVE to pay for the program but when it doesn't seem to work properly on my computer it's kinda stupid to throw money out of the window for it. Anyone who can help me with this issue? Thanks for your time and help in anycase.

Cheers

[MiRai]

Your screenshots show exactly what I was telling you in IRC (Assuming you're the same person (Maravi) from the IRC channel earlier with the exact same problem and a GTX 590) . You can see when you've got 3 slots open your GPU is sitting at 97% load (MSI Afterburner // Even GPU-Z is showing 85% load) -- You're completely maxed. When you load the 4th instance of the game you push the GPU load so far over the limit that it just cripples it and FPS take a major hit because it just cannot keep up (which is why load actually goes down in your last pic).

I had suggested that you try to lock your foreground framerate to a lower number (like, 30 FPS on all windows) and you said that you had tried that (again, assuming you're the same person, but with a different name), but in your screenshots that's clearly not the case seeing as you're getting numbers like 153 FPS and 93 FPS.

Your screenshots show exactly what I was telling you in IRC (Assuming you're the same person (Maravi) from the IRC channel earlier with the exact same problem and a GTX 590) . You can see when you've got 3 slots open your GPU is sitting at 97% load (MSI Afterburner // Even GPU-Z is showing 85% load) -- You're completely maxed. When you load the 4th instance of the game you push the GPU load so far over the limit that it just cripples it and FPS take a major hit because it just cannot keep up (which is why load actually goes down in your last pic).

I had suggested that you try to lock your foreground framerate to a lower number (like, 30 FPS on all windows) and you said that you had tried that (again, assuming you're the same person, but with a different name), but in your screenshots that's clearly not the case seeing as you're getting numbers like 153 FPS and 93 FPS.

I am the very same person yes. I also tried locking the FPS at 30 overall also - same result. Even tried down to 15 just for show off and it still screws up when the 4th slot is being loaded.

How come other people are able to run with 4 when they even got a worse GFX than I do? I simply cannot understand that.

Are you sure u are setting a fps cap in isboxer/innerspace as well?

Also maybe the fact that a 590 is a dual gpu has something to do with it.