In case RAM if corrupt or defected your PC won't start at all or if, starts it will hang while working or the computer will auto shut down with cabinet LEDs still on (the system would be running) and monitor getting on stand by with beep sound (from cabinet, if you have that small speaker inside).
When it fails to start it will provide same symptom, i.e. cabinet LEDs would be on (the system would be running) and monitor would be on stand by with beep sound (from cabinet, if you have that small speaker inside).
If the system hangs, RAM maybe or may not be a problem but, if you get above sign then your RAM needs replacement.
You're saying that without any video the PC starts off. It seems to me that RAM is nearing to it's end. You might get it scrutinised from your hardware guy.
Also, your RAM slot and RAM card may need a bit cleaning as dust also creates such symptoms.