Inspiron 24 video pixelates and computer lags

​I have an Inspiron 24 all-in-one, only 8 months old. Pretty much out of the box, it couldn't handle video or audio -- both pixelated, stuttered, broke up, and the computer slowed to a crawl. I was running Windows 10 at the time. At the suggestion of a forum query response, I tried to do a reset, but it wouldn't work no matter how much I tried. Then the HDD failed on one attempt. Thinking this may have been the problem, I replaced it was a 1 Tb SATA Seagate, and I installed Windows 7, thinking W-10 might have contributed.​

​At first, things seemed vastly improved. However, after two days the old problem came back with a vengeance. I'm at my wits end. I'm beginning to feel like I bought a $700 piece of junk. I do like the big screen, but as it stands, there's not even an HDMI plug so I can salvage it as a monitor. It does not inspire confidence in Dell. Any suggestions on what's up and what I might be able to do?​