It feels like dial-up all over again....

I am in Arizona. Last week, my modem stopped working. Thought it was because it was old, so we went to the Cox store and got a new one. Still no dice. A tech came out, and the problem was a simple fix. I was back online. Saturday, the 24th,  rolls around, and suddenly, I am suffering from lag so terrible I'm getting flashbacks of dial-up. Streams buffer constantly, YouTube videos lag, or the sound advances without the video. I didn't dare try gaming online. I've reset my modem more than once, restarted my computer numerous times, switched the coax cable, switched the modem/router location, and so far, nothing has worked. Surprisingly enough though, my cell phone and my gaming console seem to handle online just fine. This problem exists only with the two laptops in the house. My plate is full this week, and I have no time for a tech to come out. Anyone else experiencing this, or have any advice on things I can do to try and fix it myself?