You tried
all of these?
1. Check all video cable/card connections and connector pins.
2. Remove all third-party devices.
3. Reseat the video card.
4. For the Power Macintosh G4 (PCI Graphics) computer, check jumper block at J25. Make sure it is the correct color for the processor type, that it includes all required inner metal clips, and is installed in the correct direction. See "Processor Module" in Service Source Take Apart for more information.
5. Clear parameter RAM. Hold down Command-Option-P-R during startup but before "Welcome to Macintosh" appears
6. Reseat SDRAM DIMMs. Make sure DIMMs are PC-100 DIMMs.
7. Remove all SDRAM DIMMs and try replacing them one at a time to test. Replace any bad DIMMs.
8. Power Macintosh G4 (PCI Graphics): Reset Cuda chip. Refer to article 95065: "
Power Macintosh G4 (PCI Graphics) Resetting the Cuda Chip".
9. Reset the logic board:
- Power Mac G4 (AGP Graphics/Gigabit Ethernet): Refer to article 95037:
"Power Mac G4 (AGP Graphics/Gigabit Ethernet): Resetting the PMU on the Logic Board.
- Power Macintosh G4 (PCI Graphics): Refer to article 95066 "
Power Macintosh G4 (PCI Graphics): Resetting the PCI Logic Board".
10.
Test with known-good monitor. Replace monitor if necessary. Refer to appropriate monitor manual to troubleshoot defective monitor.
11.
Replace video card.
12. Replace processor module.
13. Replace logic board.