Radeon HD7850 + 3440x1440 display = black screen after boot, only when on DisplayPort

Radeon HD7850 + 3440x1440 display = black screen after boot, only when on DisplayPort. Personalities wont help...

I have Yosemite, booted with multibeast.
My current rig is:
Radeon 7850 + Dell U3415W (3440 x 1440). When I connect monitor with DisplayPort, bios loading is fine, but I get black screen on login. That's (supposedly) a well known problem, but... in my case there are some weird things:

1). When I use DisplayPort with another monitor with 2560x1080 resolution, everything is fine.
2). When I connect my 3440x1440 via HDMI, it works as well, but refresh rate is limited to either 30Hz or 50Hz (PAL - weird...).

But I really need display port connectivity, cause HDMI to connect my external TV (which does not have display port).

This might be a noob question, but I've searched a bit, and was not able to find any working solutions. So any suggestions will be highly appreciated.

UPDATE:

Kinda strange, but after computer went to sleep, DisplayPort suddenly started to work, at full 60Hz. After reboot though I had the same issue...

I'm having a very similar issue. I use a ACD 27" (2560x1440) that only has display port and it will show the apple logo and then as it's about to go to the desktop, the screen goes black. If I plug in a different HDMI monitor I have it works fine and shows the login screen.

It was working perfectly in 10.10.3.

I have an R9-290x

@randallphoto: Known issue, Hawaii-drivers are just ****. Patching a framebuffer might work, auto-init has problems with DP sind 10.10.4.

@themoon7: Do you use injection or GE=No? Try the opposite. Personally I've seen this black screen before sleep issue since 10.10.4 when using injection. Without injection everything is fine.

@Fl0r!an
Yep, I've tried all the personalities already - the ones I could find for 7*** series in a pinned neighboring thread. Experimented with GE off/on to no avail.

What really bothers me is that it works with the same ports but on monitor with lower resolution. What I've seen discussed so far is the problem that it either works or not - black or white. In my case it is kinda... gray?

I'm about to give up on this videocard at this point

Btw, manually putting computer to sleep at login screen - ctrl shift eject - doesn't not work.

Since I cannot really see any other options, there are two alternatives:

1). Reinstall Yosemite. I wonder if it makes sense - will it make any difference?
2). Get a new videocard. I was thinking about nvidia's gtx750, supposedly they will work better. At least that's my impression from reading this forum. Hence the question: if I choose this path, will I have to reinstall the system anyway, or just put the new card in and install the webdrivers?

Does anyone have any ideas?

Maybe the system is setting some incompatible resolution when you attach your 3440 x 1440 screen so it turns black? Or doesn't it get detected at all? Can you verify this through screensharing or a second display?

I dont think the issue is in incompatible resolution... then the sleep fix would not work, most likely. Anyway, I have decided to get rid of this card