Dropping GPU's NEED HELP

[itlbtu2]

  Sorry for the long post, however I have multiple mining rigs going and I had an issue with one rig dropping GPU's.  I have performed multiple trouble shooting steps.  I bought and installed new powered usb risers, swapped PSU's, and eventually bought a new motherboard etc...  (Biostar TB85) and I'm still having issues.  I am currently using SimpleMining (SMOS) I have pulled each GPU from the rack and installed them individually into my windows rig and ran  OCCT to test for card issues.  All are working properly.
  When I start the miner with 4 GPU's, all 4 will start mining properly for a several seconds to a minute and then it will drop one card, leaving only 3 GPU's mining.  I have also started each card individually and they all mine perfectly.  Also if I start two GPU's I get the same issue, works fine for many seconds up to about a minute and then only one GPU is left mining.   
  I appreciate any input you guys could give me on this issue.....I'm about to pull my hair out on this one!

[Undefined31415]

What PSU are you using, and which did you try to swap it out with?

You say this issue happens if you start two or four, did you also check 3? (Does it happen in any case you run multiple cards?)
Have you tried starting 4 instances of the mining software, each for a specific card?

[killerelite]

You should probably try to diagnose which of the part is faulty.
Since your swapped your PSU,risers and motherboard it is save to say that the only thing left are the OS and Gpus
Check your Gpus on different slots and check which one stops working (might be only one gpu stops working on different slots.
If thats the case try cleaning the pcie x16 connector and try again.
I had a problem that form 3 of my 1080 tis ,one would stop between mining after 2-5 mins , I tried almost everything ,It would work fine solo but not when put in that rig with other cards So I pulled it out and put it back in 2 days later and to my surprise it started working (and now its still working since 2 months ago)

[itlbtu2]

What PSU are you using, and which did you try to swap it out with?

You say this issue happens if you start two or four, did you also check 3? (Does it happen in any case you run multiple cards?)
Have you tried starting 4 instances of the mining software, each for a specific card?

I am running 2x EVGA 750G PSU's per MOBO.  I have an extra one not being used.  I swapped one out at a time to check for other problems. 

Yes it also happens with three.  it just consistently drops one GPU.  Not always the same GPU.

[itlbtu2]

You should probably try to diagnose which of the part is faulty.
Since your swapped your PSU,risers and motherboard it is save to say that the only thing left are the OS and Gpus
Check your Gpus on different slots and check which one stops working (might be only one gpu stops working on different slots.
If thats the case try cleaning the pcie x16 connector and try again.
I had a problem that form 3 of my 1080 tis ,one would stop between mining after 2-5 mins , I tried almost everything ,It would work fine solo but not when put in that rig with other cards So I pulled it out and put it back in 2 days later and to my surprise it started working (and now its still working since 2 months ago)

I am running simple mining OS on flash drive.....I have re-flashed it a couple times...do you think it could be a bad drive or bad flash causing this?