MBP DVI now no signal detected

2007 MBP 17" previously worked fine with BenQ 24" external monitor. Now will detect monitor but no signal detected from DVI-D input. OK VGA from my MBP and a PC. Works fine with my wife's MBA detects DVI-D no problem. Have new monitor from BenQ as thought to be fault on input port clearly not the case as other MacBook works! Have cleared RAM repaired disk permissions all to no avail. Multiple restarts and detect displays - display is selected but display detects no signal. Have now tried DVI out to HDMI - same result detects display but not the signal so nothing displays.