If the source is overloaded (as is often for EPL streams), buffering will occur... Doesn't matter if your network can handle 1080p or not..

welcome to the world of kodi!

over time, u will understand it better.

see, kodi is a platform where thousands of aspiring developers do their stuff and put the links there for others to use foc.

some do a better job than others; some maintain well, some don't care much.

add on top traffic congestion, your local broadband hiccups, country geoblocks, anything can happen.

theoretically, 10mbps can do 1080p rips smoothly but speed is only one of many requirements.

higher speeds may not necessary do better for reasons mentioned.

"tweaking" in kodi will not help a bit if the stream itself is poor.

so, it is about trying diff links in diff addons (for same video), play the best one u can find.
to that effect, understandably, many users prefer kodi as a "one stop" center - do movies, news, sports, anime, youtube, eng or other language - all in one place. but alas, that does not work very well, pretty poor for some videos.

it is for this reason one is better off using both kodi (and diff addons, links) as well as standalone apps for the same streams.

If it isn't the source, then perhaps your network isn't ideal... Sure you can increase the size of the cache & the speed at which it fills (Read about advancedsettings.xml)... For most, it works and others, well...not... In summary, the following works for me on an Android box..:
Latest stable Kodi version (Stock_no Build) + Unifi 10Mbps + Healthy network (Ethernet, etc) + Good 1080p source = Profit!

Yes you could. However powerline adapters will only work on the same electrical phase. So confirm that both circuits are on the same phase before purchase.. There's also CAT6 cabling..