Benq FP71G monitor screen displays momentarity on pc start up then goes blank. Toggle monitor power on off button to see screen for approx 2 seconds then goes blank again. Monitor cables connected and secure. Power plug connected and secure. Regards Grinnerman

Check your power supply and power cable and check your monitor cable if its not damaged..or try your monitor to another computer..

Sounds like classic symptoms of capacitor failure.
Old thread, but in case anyone else is still looking for an answer, the problem is probably bad capacitors on the power board. There was a world-wide problem with capacitors in the early 2000's that affected all kinds of electronics. For a good summary/review see https://en.wikipedia.org/wiki/Capacitor_plague .
Make sure the replacement capacitors are the proper size, specs (volts (v) & micro-farads (mf)) and temperature rated for 105°C. Make sure to install them in the proper orientation, they have a positive (+) and negative (-) terminal. I had 3 bad ones but replaced them all. It's actually quite simple and cheap if you have basic soldering skills. If you're not sure, research soldering technique and safety. Be Safe Out There.