Implausible Signal?

HI, I am having a problem with my 2001 A4 Avant 2.8, any advice would be greatly appreciated.
 
I noticed recently my car has been running in 4th and 5th gear while driving at very slow speeds (20-30mph).  This is leading to some very hard shifting.  I have an "Implausible Signal" error code up that no one can seem to figure out.  Why is my car running in these higher gears at low speeds, some times below 1000 rpms.  Any ideas would be great
 
thanks.

Post the codes.