Xbox Softmodded 1.2 Wii Softmodded white with gamecube ports wii u hacked with haxchi and vwii hacked with gamecube usb controller adapter psp 1000 cfw cipl flashed psp 3000 cfw davee's permanent patch ps2 75001 with fmcb 3ds new xl cfw 11.2.0.35 arm9 safehax
I would first try another cable to see if its the cable or not and if its not the cable I would open up the xbox and check all the connections that correspond with the outputs because I feel like a solder could be a bit loose or something lol idk thats basically what I would do.
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Consoles I own
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PS2 stock xbox original softmodded with unleashx xbox 360 stock xbox one stock psp 3000 cfw 6.61 pro-c ds original with r4 card n64 stock 2 gba sp 001 New 3ds Mario Black Edtion
Are you using it in Component on the TV or YPbPr as I have one TV that only has this instead of regular Component and mine flashes black for a second too.
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Some Of My Gear & How They Connect
Spoiler:
Dreamcast - VGA or S-Video Gamecube - Chipped RGB Scart SNES - RGB Scart PS1 - Chipped SCPH-9002 - RGB Scart PS2 - Chipped Silver Phat SCPH-50003 + 500GB HDD - Component PS2 - Aqua Phat SCPH-50003 + 1TB & SATA Adapter - Component PS3 - Blue CECH-4003C Super Slim + 320GB HDDs - HDMI PS3 - CECHG03 Phat + 500GB HDD - Component Saturn - Chipped v1 - S-Video Xbox - Chipped Crystal 1TB HDD - Component Xbox - Chipped Black 500GB HDD - Component Xbox 360 - Black Jasper - Component Wii - Soft Modded - Component Wii - Soft Modded - Wii2HDMI Wii U - HDMI
I tend to agree sounds like a faulty connection. Take the Xbox Apart. Remove Motherboard. Flip it over. Re-Solder all the pins for your AV Jack. Flip it over Also While you have it out lightly press the 2 top connector pins down on the outer shell as this helps with grounding issues which can create similar issues as you are describing.
Sometimes a 15k resister running between Composite Video Out and Component video out can fix this issue
By any chance do you use it with an earlyer model LGTV? If so, then it's a sync issue (TV's fault) which can be mitigated by powering off the TV for a few seconds and switching on/off AV mode in the quick menu.
Late to the party here but my Xbox does the same n I have an LG TV. Black flicker across a number of 'boxes n cable makes. It's not a massive issue once you get used to it.