Sometimes the CPU cooler is not properly mounted, so you need to check the CPU temperature.
If you have some issues and you don't know what it is, then the best approach is the black-box testing. To do that, remove some components, sound-cards, drives, GPU and so on. If you still have issues try another OS.
What can make Linux so unresponsive?
Overcommitting available RAM, which causes a large amount of swapping, can definitely do this. Remember that random access I/O on your mechanical HDD requires moving a read/write head, which can only do around 100 seeks per second.
It's usual for Linux to go totally out to lunch, if you overcommit RAM "too much". I also have a spinny disk and 8GB RAM. I have had problems with a couple of pieces of software with memory leaks. I.e. their memory usage keeps growing over time and never shrinks, so the only way to control it would have been to stop the software and then restart it. Based on the experiences I had during this, I am not very surprised to hear delays over ten minutes, if you are generating 3GB+ of swap.
You won't necessarily see this in all cases where you have more than 3GB of swap. Theory says the key concept is thrashing. On the other hand, if you are trying to switch between two different working sets, and it requires swapping 3GB in and out, at 100MB/s it will take at least 60 seconds even if the I/O pattern can be perfectly optimized. In practice, the I/O pattern will be far from optimal.
After the difficulty I had with this, I reformatted my swap space to 2GB (several times smaller than before), so the system would not be able to swap as deeply. You can do this even without messing around resizing the partition, because mkswap takes an optional size parameter.
The rough balance is between running out of memory and having processes get killed, and having the system hang for so long that you give up and reboot anyway. I don't know if a 4GB swap partition is too large; it might depend what you're doing. The important thing is to watch out for when the disk starts churning, check your memory usage, and respond accordingly.
Checking memory usage of multi-process applications is difficult. To see memory usage per-process without double-counting shared memory, you can use sudo atop -R, press M and m, and look in the PSIZE column. You can also use smem. smem -t -P firefox will show PSS of all your firefox processes, followed by a line with total PSS. This is the correct approach to measure total memory usage of Firefox or Chrome based browsers. (Though there are also browser-specific features for showing memory usage, which will show individual tabs).
Best Answer
atop is pretty good at monitoring and logging resource usage. It can be used interactively or as a service; the debian package sets it to log to /var/log/atop.log every ten minutes (edit /etc/init.d/atop for something more precise). You can then replay the logs with
atop -r /var/log/atop.log -b hh:mm -mM; mM selects a view and a sort appropriate for memory problems, hh:mm should be a few minutes before the incident, use tT to navigate. Also try the A sort.