Sapphire HD4870 not sending signal to monitor

Hello. I bought a new CPU E7500 for my home PC and I'm having a problem that won't let me sleep. After installing the new CPU my Sapphire HD4870 won't send any signal to my monitor, unless, ATTENTION PLZ, I unplug the PC and plug it again or I manually shut it down pressing the power off button for a few seconds. So after any restart or shutdown from software, won't work. I'm only guessing that its my videocard btw, but it's still wierd tho, as I used the HD4870 with my old CPU E4500 and there was no problem.

Sorry for the horrible english and thanks in advance.

Btw this is the configuration I'm using:
CPU: Intel Core 2 Duo E7500
Mobo: Intel DP35DP
Video: Sapphire HD4870 1gb (standard one)
PSU: Gembird 650W
Memory: 4Gb (4x1Gb) DDR2 Pc2-6400 (400MHz) Kingmax
Monitor: BenQ 2420HD