I have a 97 QX4 with 90,000 miles.  Over the last year when  travelling
at 55mph or higher, the tach will jump from about 3 to 4.5 and the car
has trouble maitaining speed.  After about 10-20 seconds the
transmission then changes gears and the tach comes back to 3 and no
problems maintaining speed.  This happens about once a week not all the
time.  It does not occur while driving aroun town under 60 mph.  Dealer
has had the car for a week at at time and can not find an error code
when on computer.  Once we recieved an error code of "knock Sensor".
Does anyone know what is wrong with this car? My feeling is that it is
the trans slipping. Somenoe told me it could be the torque converter.
What type of money are we talking to fix  Some help please.Thanks, John