2009 iMac monitor going black

[2macman]

I have an early 2009 24 iMac, and a late 2009 27 iMac.

The 27 has the common black-screen-but-computer-running problem.
The computer works, but the monitor goes black if the brightness is above 30%.
Months ago I took it to an Apple, then another repair shop. They both said blah blah $250 to change the part. I did. No fix. Nothing improved. Then they both said $500 for new display panel. I balked.

So, my solution was to run an external LG monitor from the 27.
Yesterday, in boredom, I linked the 27 to my 24 as an external monitor.
The monitor works!

The brains of the 27 will show visuals FROM another computer. And TO another monitor. But it will now show visuals from ITSELF. Baffle.

Can anyone please narrow down the problem? The monitor obviously works.

Thanks

 

[rugmankc]

Will be interested to see any solutions. I have a 2008 that goes black also and sometimes has varying blue shaded vertical lines. Have to reboot. Not sure if they are even connected being 2008 and 2009.

Maybe a reason to buy a new one--