Since the Dell U2913WM is a 2560x1080 display, and I'm currently using an Acer K272HUL, which is a 2560x1440 display over HDMI from my 12" MacBook, I'm thinking there are two possibilities, as Lanny mentioned - either it's an HDMI version problem, or an HDMI cable problem.
In this case, the U2913WM is a 2013 model year display, so it should support a new enough version of HDMI to allow connection with the MacBook. The next possibility is that either there's an issue with the multiport adapter you're using (for example, I have a VTin multiport adapter that only supports 1920x1080 over the HDMI port, even though the MacBook will cheerfully go higher) or with the HDMI cable you're using. I eventually bit the bullet and bought the Apple Digital Multiport Adapter and a brand-new, high-speed HDMI cable (Monoprice Select Series) in a 3ft length - you'll need to make sure whatever you buy is a "high speed" HDMI cable to go higher than 1920x1080. This was the only way I was able to get a stable output at 2560x1440. None of the other adapters I've tried (including Anker and Monoprice) were stable at this resolution. There are reports that "Redmere" super-slim active HDMI cables don't work well with the MacBook, but I didn't have any problems with the one that I tried.
Something else that's a recurring issue with my MacBook is that I have to blow the dust out of the USB-C port before each use - e.g., using canned air - because if I don't, the display is glitchy the way you've described.
So my recommendation (keep in mind what you've paid for it!) is:
1. Try a TV or another monitor that supports 1920x1080 resolution and see if the video display is stable.
2. Try blowing dust out of USB-C port with canned air, giottos rocket, etc.
3. Check your HDMI cable and make sure it's a high speed cable, or just get a new one like the Monoprice Select that you know is high-speed.
4. See if you can borrow/try an Apple Digital Multiport adapter.
Hope this helps.