Another possibility is that the extruder isn't hot enough to push the filament out faster for the support material.  I have ABS filaments that have different melting temperatures and softness, and on the wrong temp I get good outer perimeters or first layer, but then it gets weak on the infill until the extruder motor grinds away at the filament and the print fails entirely.  Increasing the temperature and/or slowing things down fixes this.

That was my first thought.  The default marlin firmware limits the top temperature you can interactively set via an ultra-lcd to 210 degrees (15 degrees below maxtemp).  So trying to fix this step got me sucked into making all sorts of the other firmware customizations and new features which I detailed in another post:

http://www.soliforum.com/topic/5971/fir … ed-prints/

Even after bumping the temp up to 220 or 230 my results are inconclusive. 

However, I think I know now why the nozzle tends to clog more in some parts of the print than others.  I've tested more with my questionable filament and I've noticed that clogs occur much more frequently when printing support material or large flat surfaces.  When printing vertical walls, or random shapes, it clogs much less infrequently.  I think this is because I have my slicer set to retract the filament during moves.  When printing support or flat surfaces, the head is snaking back and forth in one long continuous extrusion, and this allows contaminants to quickly build up to a clog.  When printing short intermittent bursts, the constant extruding and retracting seems to better break up partial clogs before they clog entirely.  If this is true, it would be interesting if a printing style (periodic retraction?) could be formulated either to make prints less clog-prone.

I am betting that there's some debris inside your hot end that you push up into the body, and eventually works its way back down.

Yeah, I think that's likely.  I've disassembled and cleaned out the hot end a few times, but I think my homemade filament must be periodically introducing some post-filter contaminants indicating that I might not have flushed out enough filament after switching to my new filastruder nozzle (with a melt filter). 

I haven't done enough printing to make any conclusive statements, but just wondered if anybody else had noticed some types of printing (such as very slow extrusion speeds) might be more likely to accumulate foreign material in the nozzle than others.

If I pause the printer, I find I can easily unclog it every time by poking a wire in the very tip of the nozzle without going too deep, and the filament gushes out from inside.

I am betting that there's some debris inside your hot end that you push up into the body, and eventually works its way back down.

Can you do long (5hr+) prints without support successfully?

Recently, I've been getting numerous clogs when trying to print objects with support material.   The print always starts out strong, but the support get skimpier and skimpier until the abs stops altogether.  However, it doesn't tend to clog once the printer has reached parts of the print that don't require support.

From what I can tell, when printing non-support material, the tip of the nozzle gets constantly "wiped" by the plastic below, but when printing support, a little blob can build up and harden on the very tip.  If I pause the printer, I find I can easily unclog it every time by poking a wire in the very tip of the nozzle without going too deep, and the filament gushes out from inside.

I'm using a new J-head with a .40mm nozzle printing at various temps between 210 and 230.  The clogs happen more often with filament I've extruded with my filastruder (with a melt filter), but even with store-bought filament the support is inconsistent.  Is this common?  Do I just need to bump up the size of my support material to keep this from happening?