With the mileage you have it could be one of four things.
Cheapest and easiest to replace is the TSS (Turbine Speed Sensor).
If you turn your wheels to the left and look behind the left wheel at the side of the tranny.
You will see this sensor, and the 2 wire harness connected to it.
Second thing to replace is the Range Selector Sensor.
This is on TOP of the tranny and it tells the PCM what gear the transmission is in.
If it is worn or it is LOOSE, it can cause a lot of different problems.
Third, it could be the VSS (Vehicle Speed Sensor).
This tells the PCM the axle speed (rpm).
If this is worn or failing, it will cause lots of shifting problems.
It can also cause your speedometer to act funny and irratic.
Fourth, because you have high mileage, the tranny could be giving you
warning signs that it could be failing soon.
Try the first two and see if it clears things up.
If not, then a visit to your local transmission shop is in order.
