Well, I got a little clarification from her tonight. She says it only happens when accelerating from a stop, like a prolonged hesitation. She says she tries to give it gas by pushing all the way down on the pedal and it sometimes seems to start to "kick in" as she lets up on the pedal. In the mean time, the tach does respond (the engine revs up) when she gives it gas but the car doesn't respond acceleration-wise. It eventually begins to act normally. It could happen anytime after the car has been driven a few miles and doesn't seem to matter whether it's the first time the car has been driven that day or if it's been used several times already.
As mentioned, the last time it happened it lasted for a few miles until she got to her destination (and hasn't happened again since).
I don't see how the rpms can respond to her giving it gas without the car commensurately accelerating unless the tranny isn't transferring the power. She said she tried manually shifting to lower gears while it was happening with no affect. When I got there minutes later to check it out the car ran perfectly with no indication whatsoever of the trans slipping or any poor shifting.
I'm going to switch cars with her so I can get a better idea of what's happening before I mess with anything. Are there any transmission components or cables from the accelerator linkage that could intermittently cause something like this?