Photoshop CS6 freezing on Mac OS 10.6.8

Question

Hi one of our Macs (OS 10.6.8) is freezing when we try using Marquee Selection or Feathering or Cut and Pasting in a PSD file using Photoshop CS6. Basically we have to hard reboot the Mac as we get no error message, just a locked up computer. All the other Macs are exactly the same but don't have this issue.
The file has no fonts in it, it is just an image with Layers that have no Effects. File size is 900MB.
All updates for both CS6 and Mac OS have been run and all is up to date.
Please advise.

Answer 1

Here are the base specs (serial info deleted). We're running the file off the hard drive and there are no additonal plugins. We use a standard font library but as I said, file has no text/fonts in it. It's completely locking up while using the marquee selection tool.
Cleared user preferences too when starting PS.
Gonna try a fresh reinstall and see if that fixes it, if it doesn't I think there is something wrong with the Mac itself because we have 6 more identical machines and they are not having this issue.
Hardware Overview:
  Model Name:          iMac
  Model Identifier:          iMac11,3
  Processor Name:          Intel Core i7
  Processor Speed:          2.93 GHz
  Number Of Processors:          1
  Total Number Of Cores:          4
  L2 Cache (per core):          256 KB
  L3 Cache:          8 MB
  Memory:          8 GB
  Processor Interconnect Speed:          4.8 GT/s
  Boot ROM Version:          IM112.0057.B00
  SMC Version (system):          1.59f2
Graphics/Displays:
ATI Radeon HD 5750:
  Chipset Model:          ATI Radeon HD 5750
  Type:          GPU
  Bus:          PCIe
  PCIe Lane Width:          x16
  VRAM (Total):          1024 MB
  Vendor:          ATI (0x1002)
  Device ID:          0x68a1
  Revision ID:          0x0000
  ROM Revision:          113-B9710C-238
  EFI Driver Version:          01.00.417
  Displays:
iMac:
  Resolution:          2560 x 1440
  Pixel Depth:          32-Bit Color (ARGB8888)
  Main Display:          Yes
  Mirror:          Off
  Online:          Yes
  Built-In:          Yes
  Connection Type:          DisplayPort
Serial-ATA:
Intel 5 Series Chipset:
  Vendor:          Intel
  Product:          5 Series Chipset
  Link Speed:          3 Gigabit
  Negotiated Link Speed:          3 Gigabit
  Description:          AHCI Version 1.30 Supported
WDC WD1001FALS-40Y6A0:
  Capacity:          1 TB (1,000,204,886,016 bytes)
  Revision:          05.01D06
  Native Command Queuing:          Yes
  Queue Depth:          32
  Removable Media:          No
  Detachable Drive:          No
  BSD Name:          disk0
  Rotational Rate:          7200
  Medium Type:          Rotational
  Partition Map Type:          GPT (GUID Partition Table)
  S.M.A.R.T. status:          Verified
  Volumes:
  Capacity:          209.7 MB (209,715,200 bytes)
  Writable:          Yes
  BSD Name:          disk0s1
Macintosh HD:
  Capacity:          999.86 GB (999,860,912,128 bytes)
  Available:          684.48 GB (684,475,437,056 bytes)
  Writable:          Yes
  File System:          Journaled HFS+
  BSD Name:          disk0s2
  Mount Point:          /