Black screen when starting

Problem:
Sometime starting PC screen continue black – monitor LED yellow – not green, pressing RESTART button = the same; using system floppy = the same, changing video card = the same.
After disconnecting PC, cleaning BIOS (pins 1-2, 2-3, 1-2) and/or disconnecting and replacing memory chips screen will working but another day ones again off. Some day after pressing a lot of times RESTART working again but not ever.

PC details:
Own build year 2000
OS Windows 98 SE
Generic Athlon motherboard
Processor AMD MMX Athlon 600MHz (3×200)
Chipset 750-Irongate
Memory 2x 128MB SDRAM
Video card ATI Rage 128 Expert, ATI All In Wonder 128
Sound card SoundBlaster AWE 64 Gold
Monitor Gateway EV500