Filament Guides Drag
For an easy test, try manually pulling the filament through the U-loop of guide tube. How hard is it to pull through? It should only take 1-2 lbs of tension at most. 

Then do a "tug test" on the extruder. Start it loading and grab the filament by hand to try to stop it from extruding. The Replicator 1/2/2x extruder style can typically pull ~8-10 lbs of tension and it should be fairly difficult to stop the filament. When you do stop the filament, you should hear clicking/thumping from the stepper stalling, NOT quiet grinding as the drive gear chews through the filament. If the grip slips rather than stalls, your extruder hardware needs to be tuned or replaced. 

There are three common causes of excessive feed tube drag:


Use of polyethylene tubing instead of PTFE -- this makes a shockingly large difference in drag friction. You can use a heat gun or torch (outside!!) to tell the difference between PE and PTFE. PE will soften quickly and then melt fairly easily, whereas PTFE will simply go clear and eventually char without melting. Nylon is also sometimes used for feed tubes with acceptable results, but if you have any suspicion about the tube material, just buy new PTFE tubes from a reputable vendor like McMaster. 
Too many loops/curves in the tube. Total drag friction is an exponential function of the total amount of curvature in the tube. You should have one good U-loop, no spirals or floppy zig-zags. 
Excessive friction on the spool rotation. Any spool back-tension gets greatly magnified through the feed tube per the capstan equation. This can be caused by a bad spool holder, or if the filament is knotted on the spool due to letting the end of the spool loose during handling.