RE: Dell Latitude D630 Shuts down
Open the chassis and clean the airvents and the fan assembly with a small vacuum cleaner or a can of compressed air. Reapply the thermal grease for the heatsink.
If the issue is not resolved, replace the fan assembly and check.
http://dell.to/1n3X2po - enter the service tag and download the service manual if required.
Based on your description, am not sure if you are using a notebook cooling pad or not, if not, then consider getting a notebook cooling pad - some offers on Amazon - http://amzn.to/28YYonl
If the issue persists even after replacing the fan assembly, then you might need to replace the motherboard.
You could contact our team to get a quote for a paid service call - http://dell.to/1vnT6CQ - however, I am not sure if we still have the parts required for this system model as it considered obsolete.
You could also order the parts here.
Also, please click my DELL-username and write me a private message with the service tag and your contact details(Name and Email) for case records.
Let us know if you have any other queries.