Nonresponsive throttle '98 A6 AVant V6 12V Auto

[Tihol Tiholov]

Hi, all. A friend's car once in awhile does this: the throttle pedal does
not increase RPM, it does nothing. After re-start it's fine until the next
time. Doesn't happen every day, it was a daily driver. Diagnostic codes
showed nothing. The friend thinks it might be a faulty sensor, but
basically no further clue and he's knowledgeable.

Does anyone have any ideas about the cause?

Thanks,
-- 
Tihol