Dell Quadro FX 3800 Is Locking The System

Dell Quadro FX 3800 Is Locking The System
Hi All,
just wondering if someone might have any thoughts on the problem we have.
We have Windows 7 x64 based station with Dell Quadro Fx 3800 installed.
Not that while ago suddenly card stop recognising RGB port and instead saying in NVidia control panel Display Not Derected even if nothing is connected??? Ever since we had to use DisplayPorts…
Well we could live with it, however recently PC start locking not crashing on simple 2D operations like browsing net with banners or flashing images. It’s funny really, case fully loaded in 3D contexts card never does this. The reason we think the FX 3800 is the problem because nothing like this is happening when we use old FX 3700 – another word all ok with old card.
No driver change or system reinstallation wouldn’t help.
Does anyone had any problem with Dell manufactured Pro Video cards like this one FX 3800?
Many thanks