you say it's a hardware problem. system could handle 30 browsers (at start) with max load at 70% (CPU) and 15% (RAM), but it can't respawn browsers due to lack of recources (which doesn't exist) - it seems for me to be illogically. Or i had misunderstood your message ?
No, they don't respawn at all (during few hours of scan the number of browsers just decrease). One instance works stable, but 2nd and 3d are already unstable. Current system has 6 physical and 12 logical cores (2.3Ghz each), maximum load running 3 instances is 50%, average is 30% . Tried both on Linux and windows - problem persists. System also has more than 20Gb of free ram (ddr3). Bandwidth isn't overloaded too. I don't even guess what the matter is, it's hard for me to believe this system can handle only 1-2 instances stably.
can you plase try running 3 or more scans ? On the screenshot you can see that even after 30 minutes arachni gets a problem with respawning ( in cmd is the number of phantomjs processes running (17 of 18 expected) - for easier counting)
tested on the third system (win 8.1 / x64 / i7-5960x / 32GB ddr4 /3 INSTANCES (6 browser per instance) / ) - the same situation. attached a file with logged numbers of running browsers during ~1 hour scan. i've tested on debian x64 and win serv 2016 before. Also, any of these three systems had a different internet provider. So it can't be system or hardware problem. Do you have any ideas? Maybe you should add a re-respawn in case of respawning failure?