For the past week or so, I've been trying to pinpoint a problem with my Internet connection.

Usually I blame Comcast, my ISP, but a typical Comcast outage is exactly that: a total interruption of service. I can tell from looking at the System Tray network icon that there's no connection.

This time, however, the problem was intermittent. Sometimes my connection would slow to a crawl, other times it would disappear altogether for a few minutes. But the network icon didn't indicate a loss of service.

Time for some detective work. Sherlock Holmes has a magnifying glass; I use SpeedTest. This free service runs a quick, well, speed test on your Internet connection. It's a handy way to pinpoint the source of an Internet slowdown.

First I ran it on my laptop, a new Samsung I've been road-testing the past couple weeks. Sure enough, my normally speedy Comcast connection proved very sluggish: Download performance ranged from around 2-8Mbps, and varied wildly each time I re-ran the test. Normally I see an impressive 30Mbps (or even higher).

That alone didn't help determine if the problem lay with Comcast, my router, my laptop, or something else, but this did: I then ran SpeedTest on my iPhone (which, at home, stays connected to the same Wi-Fi network, natch). The service offers free mobile apps for Android and iOS.

Lo and behold, the iPhone blazed through the test, posting the high numbers I expected. So the problem wasn't with Comcast or my router. The problem was with my laptop. Now I could focus on other possible culprits.

SpeedTest is great for this kind of process-of-elimination troubleshooting. As long as you have another device in the house, be it a smartphone, tablet, or second PC, you can compare results side by side.

By the way, my issue turned out to be a combination of Wi-Fi drivers and a Samsung-specific Google Chrome bug--but that's a hassle story for another day.

Contributing Editor Rick Broida writes about business and consumer technology. Ask for help with your PC hassles at [email protected], or try the treasure trove of helpful folks in the PCWorld Forums. Sign up to have the Hassle-Free PC newsletter e-mailed to you each week.