Last month my friends machine did much the same. I pulled the side cover and found the CPU heat sink would wobble because only 3 of the 4 retaining pins were holding. A little heat sink compound and getting the heat sink mounted proper and all is well.
This may not be your issue but a better than average PC tech would need to see your machine to see if this or other issues haunt your machine.
I purchased this machine 9-20-08, and straight out of the box Windows would not start. After hours on the phone I was able to get it started up, but it kept crashing, and had to return it for repairs to faulty graphics card that came in it.It has been working great up until this week.I leave it turned on all the time for my phone service, and I noticed that the fans had revved up, and then slowed back down, as it does when it shuts down or reboots after updates, so I flipped my TV over to HDMI2 to see what was going on. It had froze up, and I could not restart from the start menu, and had to force power off. It has crashed after a couple of minutes of start up every time since. The HP diagnostics fails my C:drive (ST3750630AS) saying SMART check BIOHD-8 error. I have checked the Seagate site to see if this is one of the Barracuda models that needed a firmware update, but it says that this drive is not effected after entering the serial number. I can not keep it running long enough to run the SEATOOLS on this hard drive, and really do not know what to do next except try a system restore from the HP partition, since my one time recovery disc burning was messed up by the initial graphics card malfunction that caused it to crash during burning at set up. Is this a HDD issue, or is there a software fix somewhere to try before restoring to factory set up?
Desktop PC with 2.66 GHz Intel Core 2 Quad Q6700 Processor, 8 MB L2 cache, 750 watt PSU
* 8192MB PC2-6400 DDR2 SDRAM memory (2x2048MB 2x2048MB OCZ gold), 2x750GB Seagate 7200RPM SATA hard drives, 500GB HP personal media drive
* NVIDIA GeForce 9500GS graphics card with 512MB dedicated video memory
* SuperMulti DVD Burner with LightScribe Technology, Wireless LAN 802.11b/g/n, LG Blu-ray reader
* Genuine Windows Vista Home Premium 64-bit