HDMI signal only works sometimes

I modded an iMac G4 to use as a HDMI display. This is fairly well documented however not a lot of people are doing this today. This connection uses TMDS so it can be re-wired directly to HDMI or DVI. I have done the modification but now I am seeing either a perfect image or total garbage depending on what source I plug it into. I have no idea why, I'm hoping someone here might!

When I connect it to a MacBook Pro via HDMI->DVI or Thunderbolt->DVI the picture works and it looks great.
When I plug it into other HDMI sources like Raspberry Pi, Amazon Fire TV etc I get random colors etc.

Clearly there is a difference in the source signal the Mac is producing. I measured the voltage coming from pin 14, 5v power, at 5.12v with the Mac and 5.23v with another source. This is being fed back into the "hotplug detect" to let the device know something is plugged in and so powering down the source turns off the display.

Other than that I can find no differences. I have used different cables and adaptors. All work with the Mac but not other sources. Anyone have an idea what's going on here? Anything I can test?

After further reading I am wondering if this is a EDID issue. The native resolution of the LCD is 1440x900. Maybe the Fire TV and the current Raspberry Pi config doesn't support this resolution but the MacBook does so the handshake succeeds?

Apparently the Pi does not support 1440x900 out of the box. Adding the following lines to the Pi's Config.txt file fixed it!
hdmi_cvt=1440 900 60 5 0 0 0
hdmi_group=2
hdmi_mode=87
config_hdmi_boost=7