First few prints were perfect, now "stuck filament detected" every few seconds  

First few prints were perfect, now "stuck filament detected" every few seconds

I recently assembled an MK4S, and had no issues in setup/calibration. The first few print were absolutely phenomenal (this is my first Prusa).

I started an hour long print (black PLA) that seemed to be going fine, so I left and returned later. The print had stopped about halfway, with the "STUCK FILAMENT DETECTED" error. I followed the unload/reloading steps, and resumed the print - the error returned every few seconds. During the few seconds of printing, I could hear a clicking from the Nextruder, which sounded similar to other printers I've had with a clogged nozzle - I think the sound is the extrusion gear or stepper motor "skipping" on the stuck filament.

I stopped the print, and did a cold pull with a different colored filament (white PLA). I got some small pieces of black colored filament from the cold pull.

I then tried to print again with the black - same error. I turned off stuck filament detection, and printing progress continued, but the result was very poor - stringing and layer shift. I could still hear the clicking noise. I used the nozzle cleaner that came in the kit.

I tried switching to the white PLA - same result. 

I'm not sure what to try next - multiple cold pulls? A new extruder nozzle? Disassembling the extruder drive and cleaning?

Any suggestion would be appreciated. I think this has to be something simple that I'm missing.

Thanks!