It'll just stop firing, dead stop. As if I stopped pressing the button. I can then repress the button and it starts firing again. That's what makes me think I might be hitting the cutoff. I take wicked long inhales. I started having the same exact problems with my Provari's, that's why I stopped using them. I'm almost certain it's my setup though.
My setup is:
SR 801 Fusion cartos in a tank. Using a 510-801 adapter 7.6-8 watts.
That's the same exact setup I used on the Provari's when it would just stop firing. Leads me to think it's something to do with the adapter(s). The silicon grommets inside might just be getting used and tired and causing problems.
Whatever the culprit it's no big deal. There's no rhyme or reason to when it does it. And it doesn't even do it that frequently, 2 maybe 3 times a day. On the Provari though it was doing it pretty much every other drag. Never happened with anything 510 though. Only when using the 801's and adapters.
I use this EXACT same setup and I pretty much run all Evolv devices (have a few - see sig line). I have not really encountered any length of hit issues so I doubt that's your culprit. I would encourage a couple of possible experiments. Every time I change a carto, I clean out the adapters and connector. Any juice accumulation "can" be misinterpreted by Evolv's short circuit protection as a possible short. This safety feature is intentionally sensitive (thus making it a SAFETY feature). Also, adapters DO fail. Over time, the internal gaskets can break down and the center post can and does start to lean to one side. So, have a look at your adapters, clean them thoroughly and sort of gently try to wiggle the center post (posi pin) and watch from each end to see if it has too much play (movement). Difficult to describe "too much" but it should barely move - sort of stiff and not move very far in any direction. If it moves easily and/or appears to have a range of movement of 50% to the negative (surrounding threaded ring), it should be replaced completely.
FWIW, I used to also have a sealed 510 to 510 adapter (in addition to the stealth 510 to 801) when Darwins first came out - as another barrier to any possible juice getting into the connector. But now most device connectors are sealed and I find the unsealed 510 to 510 offers improved airflow over just the stealth 510 to 801. So now I have unsealed 510 to 510 in all my devices and then the stealth 510 to 801. This also provides an additional bit of protection to the connector posi/center pin which can also fail and short. Finally, if you are using tanks with filler tools, which I use, I have learned it is best to remove the tank from the device before pushing the tank back down into sealed position. I had always left them on the devices for the entire filling process but occasionally there can be resistance (physical, not electrical) from stress in the internal tank o-rings and forcing it down with pressure can tear/warp the insulating gasket under the connector center/posi pin (which can also create a "short"). Now I leave the carto and tank on the device to fill and remove (unthread it) and lean it on the table to push the tank back down into sealed position (just to prevent any unnecessary pressure on the device connector). It adds an additional 20 or 30 seconds to each fill but prevents any unnecessary wear and tare on the connector.
If none of this improves your condition, I'd suggest you consider sending the device to Opus for inspection. Hope something in this post proves helpful.
Think I'll check out the dual coil heads while I'm at it . . . order coming soon ;-)
Only running about 4.2 volts on it now (9-ish watts on the Opus) since the juice I loaded in already has a strong TH, and man is it singin'!
Every time I fill it I have one of those plunger thingys that I use to re-prime the carto every time I refill it. I've become spoiled now. I'm a junkie for a fully saturated vape. 
