MakerBot Industries

I'm using 3.0mm filament, but from what I have heard the 1.75mm is more prone to jamming. You might want to try disassembling the stepstruder and cleaning out the drive gear. You may also be interested in printing out a new filament drive that makes tension adjustment and cleaning easier, such as http://www.thingiverse.com/thing:7113 or http://www.thingiverse.com/thing:7129

One thing you need to make sure is that your drive gear is absolutely 100% as aligned to the center as it can be. What I mean by this is when you look down the hole you want to see the groove of the pull right in the middle with the delrin tensioner aligned in the middle of it. This is probably the most important thing you can do.

The other thing to look for is to make sure the channels that the filament travels through are spaced just enough so that the filament easily travels through it. The tolerance on the parts varies and when you assemble it sometimes you can assemble it in a way where those channels are too big and leave too much room for the filament to buckle and bend.

One of the problems with the current setup is the long thermal barrier the filament has to travel down. It gets quite hot all along the travel distance of this barrier which in turn can weaken the filament and cause feeding problems.

What temperatures are you using?

The dual PTFE tube design is also not ideal and it probably wouldn't hurt for you to check if plastic has wedged itself between the two tubes. That will ruin it ( it happened to me once) and will need to be replaced.