Nvidia driver refuses to load for card in primary PCI-Express slot

I am having an odd problem with my Nvidia drivers. My system specs are:

Motherboard: Gigabyte GA-EX58-UD5
RAM:         12 GB
Processor:   Intel Core i7 965 @ 3.2 GHz
OS:          Windows 7 64-bit
Video cards: NVidia GTX 280 x3 SLI

Originally I was running Vista x64 and then upgraded to Windows 7 in October.  Everything was running fine in triple-SLI after the upgrade and it's been stable.  However, I just tried to upgrade my video driver from 191.07 to 195.62 (the newest WHQL version).  The driver installed *but* my display reverted to VGA mode and a little balloon popped up in the system tray that said, "Device driver software was not installed successfully."  When I looked in the device manager it showed two cards as GTX 280s and running fine, but the third card (in video slot #1) was in VGA mode with the detail message, "The device is not configured correctly (Code 1)".  Then I tried uninstalling 195.62 and re-installing the previously-working 191.07 but the same error occurred!  

So then I tried removing all three video cards and just reinstalling *one* card, in PCI-EX slot #1.  Same error after bootup: "Device driver software was not installed successfully." Next I tried just one card in PCI-EX slot #2 and it booted up and worked fine (no need to reinstall the driver, either)!  [This was using driver version 195.62.]   So then I added a second card in PCI-EX slot #3 and that also worked fine (in SLI mode as well).  But nothing I've tried can get a video card working in PCI-EX slot #1 anymore.  I even tried an old 8800 GTX card by itself in slot #1 and the same error occurred.  

After a lot of digging through the event logs I found this event in Application Log:

>>>>
Level: Information  Source: Windows Error Reporting  Event ID: 1001  Date: 1.5.2010  1:28:28 PM   Task Category: None

Fault bucket , type 0
Event Name: PnPDriverInstallError
Response: Not available
Cab Id: 0

Problem signature:
P1: x64
P2: C0000142
P3: nv_disp.inf
P4: af50922585d03da3583308f06d1d75a7b320336e
P5: Section003
P6:
P7:
P8:
P9:
P10:

Attached files:

These files may be available here:
C:\Users\dougb\AppData\Local\Microsoft\Windows\WER\ReportQueue\NonCritical_x64_ca23d0e1e31e1ccf43fe0a83aa620fd318b1352_0cc441d4

Analysis symbol:
Rechecking for solution: 0
Report Id: 1e2cc5ae-fa28-11de-96fc-001fd0ae7c1d
Report Status: 4

===========

Detail XML values:
- System
  - Provider
   [ Name]  Windows Error Reporting
  - EventID 1001
   [ Qualifiers]  0
   Level 4
   Task 0
   Keywords 0x80000000000000
  - TimeCreated
   [ SystemTime]  2010-01-05T18:28:28.000000000Z
   EventRecordID 24111
   Channel Application
   Computer sookie
   Security

- EventData
   0
   PnPDriverInstallError
   Not available
   0
   x64
   C0000142
   nv_disp.inf
   af50922585d03da3583308f06d1d75a7b320336e
   Section003
   C:\Users\dougb\AppData\Local\Microsoft\Windows\WER\ReportQueue\NonCritical_x64_ca23d0e1e31e1ccf43fe0a83aa620fd318b1352_0cc441d4
   0
   1e2cc5ae-fa28-11de-96fc-001fd0ae7c1d
   4
   
================

Here is the detail file mentioned in the event in the 'C:\Users\dougb\AppData\Local\Microsoft\Windows\WER\ReportQueue\NonCritical_x64_ca23d0e1e31e1ccf43fe0a83aa620fd318b1352_0cc441d4' directory:

Version=1
EventType=PnPDriverInstallError
EventTime=129071897089677734
Consent=1
ReportIdentifier=1e2cc5ae-fa28-11de-96fc-001fd0ae7c1d
Response.type=4
Sig[0].Name=Architecture
Sig[0].Value=x64
Sig[1].Name=Win32 error
Sig[1].Value=C0000142
Sig[2].Name=Inf name
Sig[2].Value=nv_disp.inf
Sig[3].Name=Driver Package hash
Sig[3].Value=af50922585d03da3583308f06d1d75a7b320336e
Sig[4].Name=DDInstall section name
Sig[4].Value=Section003
DynamicSig[1].Name=OS Version
DynamicSig[1].Value=6.1.7600.2.0.0.256.1
DynamicSig[2].Name=Locale ID
DynamicSig[2].Value=1033
FriendlyEventName=Could not install driver software for device
ConsentKey=PnPDriverInstallError
AppName=Driver software installation
AppPath=C:\Windows\System32\rundll32.exe
   
<<<

I've done a lot of hardware troubleshooting over the years, but for the life of me I can't figure out why the driver now just refuses to load for any card plugged into PCI-EX slot #1.  [I didn't notice anything particularly helpful in the event log.]  Currently I am running just fine with two cards in SLI mode in slots #2 and #3 using the latest 195.62 driver, but the driver just *refuses* to load for any card plugged into slot #1 even with no other cards in the system (I tried a different GTX 280 as well as an 8800 GTX, still no dice).  Any ideas???  I'm stumped on this one.