UPDATE:
I found the root cause. Only thing I changed during reinstalling & upgrade was the graphics card.
I wanted to upgrade from traditional generic ATI card to Quadro FX-series card and after several try-outs
(includes reinstalling cubase & asio driver / running system without asio, firewire cables removed etc…)
I realized that my power supply was running out of power! I switched back to my old graphics card and everything
started to work smoothly.