Lots of good suggestions, but very few of them have even bothered to start with the recommendation of TESTING FIRST. I do see a couple suggestions to clean, which would be my first suggestion.
Before you start throwing more money at it with new hardware, I would start doing some initial testing as well as some thorough cleaning. In cases like this, heat is almost always a factor, and if the fan/heat sink have not been cleaned and checked in a long time, then airflow is definitely part of the problem. Also, make sure that the surface you're using the laptop on is conducive to proper airflow.
Here's what I would do:
1 - Check that the cooling fan (or fans) is working properly. You should be able to hear it spinning and hopefully pushing/pulling air
2 - Check the heat sinks/vents to make sure air flow is good. You should be able to place your hand next to the outflow vent and feel warm or hot air exiting.
- If either of the above indicates a problem with airflow/cooling, then you need to remedy that before you try ANYTHING else. It's entirely possible that the fan in your 6-year-old laptop is simply aging and not moving enough air.. and it's also entirely possible that it works fine, but dust has completely blocked the air it would be moving.
3 - Assuming air flow is not the issue, you need to start by completely circumventing the currently installed O/S and hard drive. Get a 8GB or 16GB flash drive, install Linux Mint or another Linux distro on it. Alternatively, if your laptop has working CD/DVD drive, get ahold of a Linux bootable CD/DVD and use that. Boot your laptop from the CD/DVD/USB so that it is not running at all on the hard drive, and see if it handles running Linux from the removable media without issue for a decent length of time.
- If it works fine with that, then it's likely either your hard drive or the O/S installation on it.
- If it still locks up or crashes running Linux from the removable media, then it's likely there is a physical hardware issue, like RAM or the motherboard/CPU/chipset.
