Some times monitor goes on & some time blank

Hello sir,

Please help, after ideally keeping my system for 8 months. While now putting on my cpu, it starts processer runs but monitor goes blank. After cleaning ram my system worked. But another day it happend like earlier. My monitor goes blank like DOS with a single mark like this _
& cpu sounds a single beep.

I have removed battery & ram then clean them well. Nothing happens. I have P4 system with 1gb ram