Replace the Teflon / PTFE / bowden tube
I ordered more Teflon tubing before running out of the original two pieces I received with the printer.They had to be shortened regularly because they were getting damaged by traction in the quick connectors, and instead of popping out, some even was torn apart in the filament feeder plug !
The replacement I bought was a "teflon tube 4mm inner 6mm" (well, there is only one way to read the diameters). I got 12 ft for $19 with shipping on eBay.
Actually, I used it immediately because it is stiffer and it has thicker walls. It will not bend as easily as the original bowden tube I received, and the few pinch marks I got were harder to smooth out.
It also seems slipper, so no way I'd go back to the first tube.
The main drawback is that it is translucent, i.e. not as transparent as the original one. Because of this, it is not easy to see the filament moving within.
I also will have harder time when I want to add a "spy" on it, like an optical mouse sensor, to let the firmware know when it stalls.
Finally, I highly recommend that you bevel the extremities of the tube and use this very efficient clamp.