Sorry bro no disrespect but been doing this for a long time
Just because you've been in this for awhile doesn't mean you're an expert. There's JL, Alpine, etc... certified installers with less knowledge than the average person into audio.
If you're playing it loud while at a stoplight, the voltage coud be dipping below 11V and sending the amp into protect.
Assuming that the amp is running at about 70% efficiency putting out 1000RMS, 1000RMS/12.6V/0.70 ~ a potential 115A draw on the battery (neglecting alternator voltage for simplicity because they don't sag linearly), assuming about 15ft of 4 gauge wire, you can calculate that your setup could cause 1V of voltage sag, that could potentially put your amp into protect once you factor in other feature on the car draining amperage too. Most amps kick in at about 11V.
400W made my Blazer's electrical sag to about 11.6V under really heavy bass at idle, so it's a good possibility that your electrical isn't keeping up.
Combine all of that, and it could be that you need to do the Big 3 upgrade, upgrade your alternator to handle the current, or turn it down at stoplights.
In the audio world, we have a generic assumption that most factory alternators can handle about a 1000RMS draw on them without needing electrical upgrades. So you may be reaching the limits of your electrical right now.