Hi Exuno and welcome to the forums!

first off... does it actually work to have 2 video cards piping video into 2 different monitors?? i wondered about this, this weekend.. was told it wasn't...

back to the issue... keys can start to get dropped if your system gets over taxed. here are some things you can do to improve your cpu/gpu performance:

o try dropping the video resolution
o try dropping the video quality on your clones (you don't need the detail anyhow)
o set maxfps to 40, and set maxfpsbk to 15 (adjust to find balance point)
o get more system memory.. 3G is about all XP can recognize

keep in mind, even if you cpu says it's only at 80%, on a dual-core you could have 1 cpu at 100% and the other at 60%.

if you can run with 3 without a problem... and adding 2 more starts to lose keys.. odds are its the weight on the system

let us know how it goes. g'luck