As you noticed yourself about your specs, I would say the problem is the 5 GB of RAM. You don't mention which OS you are using: a 32 bit version of the OS would typically limit user-space RAM to about 3 GB - that's a bit short for modern apps, especially while running a full node.
What you described (in bold in my quote) is probably caused by what I call "swapping to death". It's the symptom of the OS running out of memory, and because of this, it is forced to swap memory contents back and forth from/to the hard drive, which is about 3 orders of magnitude slower than RAM. This may cause your machine to become completely unresponsive for several minutes.
I always wondered why OS designers wouldn't implement a system warning message whenever the system needs to use the swap file to such an extent that the user experience becomes heavily degraded. Most users don't understand what's happening in the background, and what they can do about it (i.e. add more RAM)
Ah, good point, I didn't mention the OS. I'm using Windows 7 64 bit, so it should totally support the 20 GB of RAM that I'm planning on upgrading to eventually. Thanks for the technical explanation of what's going on, so it sounds like the RAM upgrade will probably fix the issue then. Unfortunately, the motherboard on my computer only has two RAM slots, so I'm going to have to get a 16 GB chip to upgrade to the 20 GB of RAM that I'm aiming for. (The one on my Amazon wishlist is going for about $171 right now. It was actually significantly cheaper (like $20 - 30) on bitcoinstore.com last I checked though, so I'll probably buy it there.)
You're right, it would be smart for OS designers to warn users about this kind of swap file need. I just hope that this reliance on swap files that's gotten a bit more common lately isn't going to degrade the lifespan of my hard drive.