DELL PRECISION T3400 FREEZING CONSTANTLY

Set up: Windows 7 x64 Professional (SP1)

             Intel Core 2 Quad Processor Q9550 2.8Ghz

             8GB Crucial RaM (New)

                ASUS AMD Radeon HD6450 Graphics card

             Corsair cx750 PSU

             M Audiophile 2496 audio interface

             4 x Seagate HDD

             BIOS A14

I have been upgrading my Dell Precision t3400 over the last couple of months, starting off with RAM upgrades, Graphics card, Hard drive updates etc. All has been running very smoothly. I wanted to boost the CPU power recently and purchased an Intel Core 2 Quad Processor Q9550 2.8Ghz to replace the Core 2 Duo E4300 that was fitted as standard. Various forums suggested that this would be ok.

 I also upgraded the heatsink to the XP850 as recommended, and replaced the thermal compound.

 Once I had changed the cpu Windows booted fine, and it detected the CPU with no problems. However, when it came to doing any slightly straining tasks (e.g transferring large files, internet browsing etc) it would completely freeze up, and I would have to pull the plug to shut it down. I thought that I might need to upgrade the PSU, which I did to a Corsair CX 750. Size wise it wasn’t a perfect fit, but I managed to improvise to secure it in the machine. However, I am still having the same problem.

 Action taken so far to fix this:

 -          Upgraded all drivers for everything.

-          Checked I have the latest BIOS version

-          Fresh install of Windows

-          Checked cpu temps. All fine at around 40 degrees while idle.

 Nothing so far has worked, and i am at the end of my tether. Please can someone assist?!