How to expect PowerLine Networking to work

I've had a mixed homeplug network consisting of a 500mbps/4 port gig-e switch and a pair of 200 mbps non passthrough adaptors. Its reasonably similar to yours I've found a few things troubleshooting it. You can see a somewhat stripped down network diagram in my answer here

Firstly that not all adaptors in the network behave identically - the gig-e switch is significantly more sensitive to noise (though I have no idea to whether this is due to it being in a mixed network, or inherently sensitive to noise). While useless in most cases if the signal lights report worse than perfect signal, you're probably alright on the power line side of things. Oddly I found its more likely something is wrong with your devices report a perfect signal, but you have connection issues.

As for noise, the only real way to work this out is to go on walkabout - my switch goes down whenever one of our washing machines run (the 200 mbps gear has signal degradation). The troubleshooting methods available on homeplug gear is rather useless - in my case the switch reports perfect signal (Its usually red or amber) when it isn't working, and occationally blinks. I started by switching off all other devices in my room, ruled out any changes in the next room and the next when the networking was glitching, and worked my way through the apartment. Focusing on devices with big motors like Fridges and washing machines is a good idea. You can probably add a filter, but I haven't found one for my local plug type. Some people actually use passthrough homeplugs for the same thing, but it seems wasteful. I ended up fixing this issue by moving the homeplug connection further from the devices that gave me issues, and I have had no issues since.

I'd also 'check' to make sure all the homeplugs are on the same private network name - they generally tend to be set to HomeplugAV, but changing it would be a good idea. You can set it with magic everything button Homeplug units have, or use a utility provided by the company that makes them (which tend to not actually tell you what the current network name is, or actually provide any sort of feedback whatsoever). In theory you can also detect what devices are on the network, but practically this works rarely enough (even when I'm on the network, and connected properly) that its pointless.

In your case it was something else, but I'd like to put this out there since anyone looking for my problem or something similar

I recently bought a Netgear powerline ethernet adapter that was capable of 300Mbps data transfer and should (theoretically) push my full 105/12 Mbps Comcast service. I actually get roughly 35-40Mbps, which sucks to have to settle for that kind of loss, but the reality is that you're hard-wired with that speed. Barring any major electrical noise, your latency will be super-low and the signal will be consistent. My ISP's .ac router has the range to reach where I'm at, with fair signal, but there's too much signal power loss (best case scenario = ~55dB... OKAY at best). I ran into intermittent signal loss that would screw with my web browsing and my online gaming at times - super annoying!

Jumping to a powerline adapter greatly improved my connection, even at the loss of speed. To get more to the point, the more stuff you have plugged into your walls across the house, the more opportunity for signal loss due to the noise on of those currents. If you have a ton of stuff that you leave on for no good reason, get in the habit of turning them off. I found that my high speed USB phone charger for my Nexus 6 and my Xbox One were creating a lot of noise that greatly degraded the signal from 40 to as low as 5! Now, I usually can leave my Xbox One on, but power cycling it ramped up my speed instantly, so I knew that was one item that (if left on for many many hours at a time) is capable of bottle-necking my powerline adapter.

As stated earlier, even older wiring can support good speeds, but remember that your wiring can run hundreds of meters depending on how everything is setup, and jumping between circuits further drops your signal - enough to probably cut your bandwidth by more than 50%. So if you don't have a fast internet service (under 15 Mbps), powerline adapters could ultimately be pointless, since you could see subpar connectivity. However, newer homes with up-to-date wiring can prove to offer lower noise when using these adapters. My home isn't new... not old, but not new. A newer wiring setup would probably double my speeds.

In the end, if you're choosing between accepting weak WiFi signal and decent, if reduced, powerline speeds, choose the later for sure. If you'd rather, run a powerline adapter from your source and put it maybe mid-way between your modem/router and where you need internet then hook up an access point. You'll probably pull the best speeds that way (if you aren't required to have a hardline connection). It just costs a bit more since you need the adapters AND the AP for that.