For the past x number of weeks every time iv'e booted tf2 to play i keep getting connection troubles followed by graphical errors leading to a crash to desktop and loosing connection to the net full stop, i only get problems when i start up TF2. Playing BC2, killing floor, l4d for hours on end with no problems.

Any ideas what could be causing this?

i have this sometimes, it lead to a graphics card update for me but im not if that would work again (old graph card)
Hope it helped.