MultiMiner having problems with GPU on one of my computers

Hi.
A few days ago I've built a computer with some spare parts (Asus Z87I WS, i5, 16 GB RAM, Gigabyte R9 270X 2 GB, Win7Ult, Catalyst 13.12) but I have a problem with MultiMiner (my preferred mining application).
The application detects my GPU but as soon as I set the optimal settings (--thread-concurrency 15232 -w 256 -I 19), settings that worked perfectly with the same GPU on another computer, mining doesn't start. If I leave the settings field blank or I set the thread concurrency to a low level (e.g. 1024) the GPU starts mining but just with 6-7 kH/s.
I tried to use GUIminer scrypt (with CGminer) with the optimal settings on the same computer and it works giving me about 450 kH/s.

What could be the cause of this problem?
I'm working with MultiMiner on my main mining computer (with the same OS) and it is working perfectly.

Thanks.

Bye
lcettuzzi