Use whatever key gets you to BIOS. Does video work well in BIOS?
YES, BIOS video is fine.
If VGA connected then reduce the frequency of the output monitor -- sounds like it's losing sync. Come up in 'SAFE' mode (google it) then reduce frequency in video settings. (google change 'refresh rate')
Possible driver problem. Come up in SAFE mode, install video driver.
NO, BIOS video is bad:
Check cables. remove then put back on. swap with other cable. swap with other monitor.