Like many consumer-grade devices, the workhorse Archer C7v2 (I own five of them) isn't built with military- or automotive-grade components. Assuming you've got a sufficient "wall wart" (1 A minimum, 1.5 A preferred, assuming no USB devices, with a 5.1 mm plug, as I recall -- the 5.5 mm plugs can be intermittent), it is possible that the power supply within the unit is beginning to fail.
The Archer C7 was a great router in its day, but that day is long past. Even at US$40 used, I think that there are better options out there.
SQM for rates over a couple hundred mbps is typically beyond what a single-core, MIPS-based router can handle. For a concise set of recommendations to start looking at, I find the post below and those that follow in the thread a good starting point. (Ignore the title, the post was a response to the flow of the thread, not its title.)
There are a couple "What's Your Favorite..." threads as well, but the above is a good starting point, in my opinion.