It could be any of the sensors, but it the only way to really tell is to have a scanner attached to the car and taken for a test drive. "A Scanner" being one that actually reads the data stream to and from the PCM. If you don't have one, such as
this one for an OBD I system, you will have to have a shop do the tests for you. Look at paying around $45 for the tests.
If your not getting a CEL or Flashing O/D light, then that rules out the only other thing it could be, which is a slipping torque convertor.
