I have a heat sink and always SSH but like others are suggesting its probably an undervolt issue although its feeding directly from USB power port. Might be the long cables I use
I'll take a guess at what they meant, and offer an explanation.
All wires have resistance, and all else being equal, a 2x longer wire will have 2x more resistance. Due to Ohm's law, there is a voltage drop in the cable when you are powering a load (such as the RPi), and this drop will eventually go below the minimum for the load.
Example: the RPi needs 5 volts at 2 amps.
If your cable is 0.1 ohms, then the voltage at the Pi will be 5 - (2 * 0.1) == 4.8 volts.
If your cable is 1 ohms, the voltage at the Pi will be 5 - (2 * 1) == 3 volts.
But because the Pi doesn't always need the full 2 amps, that 1 ohm cable will work sometimes, making debugging harder. Only when you try to do something compute and/or RAM intensive will you notice the board randomly die.
This is right. The way I have setup my home office I need a 3x wire cables for my raspberry pis and this is probably causing an undervolt. I might have to connect a display/vnc and check for warnings.
The LED light does go steady red which is an indication that it has conked off and needs a hard reboot
Make sure the USB port can pump out enough current. Make sure you use a short, thick USB cable. Once you do that, stability issues are gone.
I also put a heat sink on the CPU, but it probably has no effect on stability. It reduces the amount of time the CPU spends in thermal throttling.