BSOD when system sitting idle

I have a Dell E6500 running Windows 7 Professional, 64-bit system. It's been running fine for months, but suddenly within the last 48 hours I've had three BSOD's, all when the system was sitting idle. (Programs active on the screen, but I wasn't physically using the system when the crashes happen.)

I spoke with Dell tech support on the phone, and we ran the built-in diagnostics on the system, and all the diagnostics pass.

I haven't installed any new hardware/drivers/whatever recently, although I can't speak for anything Windows Update might have done to my system. (Nothing obvious based on looking at restore points in System Restore.)

The three faults seem to be the same:

Probably caused by : ntoskrnl.exe ( nt+71f00 )
Followup: MachineOwner
---------
1: kd> !analyze -v
*******************************************************************************
* *
* Bugcheck Analysis *
* *
*******************************************************************************
UNEXPECTED_KERNEL_MODE_TRAP (7f)
This means a trap occurred in kernel mode, and it's a trap of a kind
that the kernel isn't allowed to have/catch (bound trap) or that
is always instant death (double fault). The first number in the
bugcheck params is the number of the trap (8 = double fault, etc)
Consult an Intel x86 family manual to learn more about what these
traps are. Here is a *portion* of those codes:
If kv shows a taskGate
use .tss on the part before the colon, then kv.
Else if kv shows a trapframe
use .trap on that value
Else
.trap on the appropriate frame will show where the trap was taken
(on x86, this will be the ebp that goes with the procedure KiTrap)
Endif
kb will then show the corrected stack.
Arguments:
Arg1: 0000000000000008, EXCEPTION_DOUBLE_FAULT
Arg2: 0000000080050031
Arg3: 00000000000406f8
Arg4: fffff80002a52798
Debugging Details:
------------------
***** Kernel symbols are WRONG. Please fix symbols to do analysis.
*************************************************************************
*** ***
*** ***
*** Your debugger is not using the correct symbols ***
*** ***
*** In order for this command to work properly, your symbol path ***
*** must point to .pdb files that have full type information. ***
*** ***
*** Certain .pdb files (such as the public OS symbols) do not ***
*** contain the required information. Contact the group that ***
*** provided you with these symbols if you need this command to ***
*** work. ***
*** ***
*** Type referenced: nt!_KPRCB ***
*** ***
*************************************************************************

(many repeats of the above box omitted)

ADDITIONAL_DEBUG_TEXT:
Use '!findthebuild' command to search for the target build information.
If the build information is available, run '!findthebuild -s ; .reload' to set symbol path and load symbols.
MODULE_NAME: nt
FAULTING_MODULE: fffff80002a18000 nt
DEBUG_FLR_IMAGE_TIMESTAMP: 4a5bc600
BUGCHECK_STR: 0x7f_8
CUSTOMER_CRASH_COUNT: 1
DEFAULT_BUCKET_ID: VISTA_DRIVER_FAULT
CURRENT_IRQL: 0
LAST_CONTROL_TRANSFER: from fffff80002a89469 to fffff80002a89f00
STACK_TEXT:
fffff880`009f4c68 fffff800`02a89469 : 00000000`0000007f 00000000`00000008 00000000`80050031 00000000`000406f8 : nt+0x71f00
fffff880`009f4c70 00000000`0000007f : 00000000`00000008 00000000`80050031 00000000`000406f8 fffff800`02a52798 : nt+0x71469
fffff880`009f4c78 00000000`00000008 : 00000000`80050031 00000000`000406f8 fffff800`02a52798 00000000`00000000 : 0x7f
fffff880`009f4c80 00000000`80050031 : 00000000`000406f8 fffff800`02a52798 00000000`00000000 00000000`00000000 : 0x8
fffff880`009f4c88 00000000`000406f8 : fffff800`02a52798 00000000`00000000 00000000`00000000 00000000`00000000 : 0x80050031
fffff880`009f4c90 fffff800`02a52798 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : 0x406f8
fffff880`009f4c98 00000000`00000000 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : nt+0x3a798

STACK_COMMAND: kb
FOLLOWUP_IP:
nt+71f00
fffff800`02a89f00 48894c2408 mov qword ptr [rsp+8],rcx
SYMBOL_STACK_INDEX: 0
SYMBOL_NAME: nt+71f00
FOLLOWUP_NAME: MachineOwner
IMAGE_NAME: ntoskrnl.exe
BUCKET_ID: WRONG_SYMBOLS
Followup: MachineOwner
---------
1: kd> lmvm nt
start end module name
fffff800`02a18000 fffff800`02ff5000 nt T (no symbols)
Loaded symbol image file: ntoskrnl.exe
Image path: \SystemRoot\system32\ntoskrnl.exe
Image name: ntoskrnl.exe
Timestamp: Mon Jul 13 19:40:48 2009 (4A5BC600)
CheckSum: 0054B487
ImageSize: 005DD000
Translations: 0000.04b0 0000.04e4 0409.04b0 0409.04e4
1: kd> lmvm nt
start end module name
fffff800`02a18000 fffff800`02ff5000 nt T (no symbols)
Loaded symbol image file: ntoskrnl.exe
Image path: \SystemRoot\system32\ntoskrnl.exe
Image name: ntoskrnl.exe
Timestamp: Mon Jul 13 19:40:48 2009 (4A5BC600)
CheckSum: 0054B487
ImageSize: 005DD000
Translations: 0000.04b0 0000.04e4 0409.04b0 0409.04e4
1: kd> lmvm nt
start end module name
fffff800`02a18000 fffff800`02ff5000 nt T (no symbols)
Loaded symbol image file: ntoskrnl.exe
Image path: \SystemRoot\system32\ntoskrnl.exe
Image name: ntoskrnl.exe
Timestamp: Mon Jul 13 19:40:48 2009 (4A5BC600)
CheckSum: 0054B487
ImageSize: 005DD000
Translations: 0000.04b0 0000.04e4 0409.04b0 0409.04e4



Any help would obviously be greatly appreciated. I'm really hoping I don't have to melt this machine down - it's only 3 months old...

Kevin