<p>I have a Printrbot Simple Metal. The extruder is getting clogged all the time. I went through the process demonstrated <a href="https://www.youtube.com/watch?v=L_qP5AsnQNQ" rel="noreferrer">here</a> multiple times already. Heatting the extruder and pushing different tools all the way through to make sure it is completely clear. 
Every time I get a clear flow of PLA, and after a few minutes the extruder motor starts clicking again. At that point, it is even hard to push in the filament by hand.<br>
I replaced the tip already, but this didn't make any difference. I also tried few different filaments, all of them worked perfectly before.<br>
It feels like stopping the flow even for about 30 seconds would cause it to jam.</p>

I'm not sure if this is particularly the issue for you right now, but I have encountered the tension on my drive gear being too high.
Before I upgraded my extruder to the spring loaded mechanism, my extruder used a Delrin plunger to provide tension against the filament towards the drive gear. This plunger used copper washers to help adjust the tension required. Many people ended up upgrading their assemblies to the spring loaded mechanism as the plunger would either put too much or too little tension against the drive gear. Too little and the filament will not be driven into the hotend. Too much and the filament will grind against the drive gear and the drive gear may begin to "eat" away at the filament (especially when the filament becomes hotter). Obviously, if the drive gear is no longer able to catch on the filament, the nozzle will get clogged.
However, even with a spring mechanism, your tension can become too high. Most of these mechanisms allow you to adjust the tension by tightening/loosening the screw holding down the spring. The "sweet spot" for me is just past when the filament can be successfully driven into the hotend. You can test this if you have control over your extrusion motor by removing the extrusion motor from its mount, leaving the spring mechanism installed on the face of the motor. Then, turn your motor on and try feeding the filament through the mechanism. Starting with no tension on the spring, begin slowly increasing tension by tightening the screw on your mechanism until the drive gear is successfully able to guide the filament through. I might even complete another half turn on the screw to account for varying diameters and plasticity states of the filament as it becomes hot.
Hopefully this helps and please keep us updated on anything else you find.