Thank you all so much! It looks like the problem isn't in the feeder, given your guidance.
Here is what is happening: When using ABS, when we stop a print (pause OR finish one print and move on to another with cooling in between OR finish one item on the print bed and move to another item on the print bed without cooling in between), the nozzle clogs when printing the next item or resuming printing. Sometimes it is a total clog (no filament extruding), sometimes a partial clog (a bit of filament sort of extruding willy nilly usually sideways). Going through the filament replacement process solves the problem (sometimes supplemented by cleaning off the nozzle if it was a total mess). However, when it prints again, the first few minutes will print yellow (burnt). Even when you replace with the exact same filament that was in there before, and the filament is white going in, it still extrudes burnt for a bit. And the area right above the print nozzle also smokes a bit during printing, so I'm guessing it has something to do with heat.
Question 1: Any suggestions?
Question 2: We had hoped to move between filaments (part of the reason for purchasing the Ultimaker). You seem pretty adamant that I shouldn't move back from ABS to PLA, at least not with the same nozzle. Why? What are you anticipating/suggesting?
Note that we have access to a MakerBot, so can print PLA elsewhere if needed, but one of the reasons for getting the Ultimaker was to have ABS and HIPS as an option.