X51, multiple error messages, show intermittently

**Model: Alienware X51 - for further details about the model, please see at the bottom of this email. 

 

- I intermittently keep getting one or the other message:

(1) "Internal hard disk drive not found. To resolve this issue, try to reseat the drive"

-or-

(2) " Error Code 2000-0123. Validation: 5559. MEMORY INTEGRITY TEST DISCREPANCY". 

- Recently updated RAM to 8GB: ( Timetec Hynix IC 8GB Kit (2x4GB) DDR3 1333MHz PC3-10600 Unbuffered Non-ECC 1.5V CL9 2Rx8 Dual Rank 240 Pin UDIMM Desktop Memory RAM Module Upgrade (8GB Kit (2x4GB))

 

- After two days of working fine, my PC self-restarts, prompting either message (1) or (2). 

 

- I ran ePSA: everything worked, but Memory. I reseated RAM slots between each other. Ran with just one x4GB slot- it worked for about 20min. Then PC restarted itself, and I had "HDD not found" message again.   

 

- Disconnected HDD, and reseated to ensure everything is well connected. 

 

- Still does not work. 

 

Can someone kindly assist, please?

Cheers!

 

**Model Details

Alienware X51 Matte Stealth Black Chassis, English Genuine Windows® 7 Home Premium (64 BIT)

Video Card	1GB GDDR5 NVIDIA® GeForce® GT 545
Hard Drive	1TB Serial ATA (7,200 rpm)

Processor

Intel® Core™ i3-2120 (3.30GHz, 3MB L3 Cache, 2C)

Power Supply

240W AC Adaptor

Memory

4GB 1333MHz (2x2GB) Dual Channel Memory

 

__