Having cyclic underextrusion. Suspect bontech gear malfunction  

Having cyclic underextrusion. Suspect bontech gear malfunction

This is an unmodified or upgraded MK3 machine. After years of printing without issues, I am now having a weird, cyclic underextrusion problem that I suspect may be related to some malfunction with the bontech gears. As the first layer is being laid down, it exhibits a "banding" pattern, alternating between what appears to be a normal thickness band, followed by an underextruded or thinner band, then this repeats. Each of the two bands appears to be approximately the same width and can be readily felt by sliding a finger over them. As the first layer continues and each trace gets longer and longer (part geometry dependent), the bands get progressively narrower and narrower, seemingly in sync with how much filament is extruded during one revolution of the bontech gears. Oddly, the bands have sharp edges, as if, at some point during the gears' revolution, they meet a step-function friction component increase that continues for half a revolution, and then, just as suddenly, returns to its normal, value, whereupon the layer returns to its proper thickness. This same behavior happens with all  filaments. 

When I removed the pressure door, it appeared that the axel of the non-driven (idler) gear had possibly worn an "egg" shaped hole in the pressure door. I did not think this to be unusual, given the number of hours of printing on the machine. What did seem odd is that this problem was not present one day and appeared all at once the next time I used the machine. I cleaned the compartment to make sure there was no filament residue inside or on the gear teeth. The gears themselves do not appear to be worn or damaged. I printed a new pressure door and reinstalled the idler gear which did not exhibit any "slop" in the axel hole. The banding/underextrusion problem continues. The attached photos shows the print after the first full layer. Note that when the nozzle gets to the central hole, shortening the line length until it restarts "full" width lines on the "other side", then the band lines get wider, corresponding with a lesser rotation of the gears. Once the hole is passed and the "long" lines resume, the bands get narrower again. (Pictures attached out-of-order, sorry.)

The next picture shows the result of the second layer in the reverse direction: short excursions at layer start - wide bands. Long excursions at layer end - narrow bands. When the bottom layers are complete and the infill starts (honeycomb), the "cells" of the infill are poorly formed and do not reinforce each other. After several layers of infill, I stopped the print. The whole print feels very "spongy" and is not structurally stable (twists easily).

A first layer calibration test gives inconsistent results. For a given Z-offset value, the line width varies and the final rectangle is incomplete. Will new bontech gears solve this problem? What other solutions should I explore?