I found some info that the 3 readings are cpu, socket and northbridge, but i don't know in what order. Google around for it.
Reading the GPU temperature might require extra software, not all readings are coming from lm_sensors. Take a look at psensor's website.
So let's rule out the temperature first. I'll assume you're on ubuntu or possibly mint. We'll use the commands from this article to try and read what the cpu temps are. http://wpitchoune.net/blog/psensor/
sudo apt-get install lm-sensors
sudo sensors-detect
sensors
If you're getting very high like 80c then the computer is probably shutting itself off. See if you can get sensors to run and then keep an eye on the temperature. If the temperature isn't getting high then we'll have to try some other things.
Also while you're at the terminal can you type uname -r so we can get an idea what distro and version you're running.
That's another issue, I'm running Linux right now as this computer also acts as my file server. I can't find a suitable program that acts like HWMonitor in Windows. The only decent program I got working is PSensor.
I've never seen /proc/acpi/ibm/thermal on mine either and it was fine... do you have lmsensors installed? Also, I would consider 40 to be quite normal back when i had the 240 once it had been on longer than 5 mins - 43-47 was a decent idle temp.
You can use the graphs produced by psensor to get all your rebound zones for the fan nailed as it plots cpu load against fan speed against core temp.