Ditto on the IC pump. I went with the Johnson as well (hmmm that sounds bad).

BUT, I also has the same symptoms and it turned out I also had a bad accelerator pedal value sensor. Since our cars are "drive by wire" when it comes to the gas pedal, if the potentiometer in the pedal goes bad, it can send bad info to the throttle.

One way I tested mine was to monitor the TPS values with my OBD app on my iphone. Driving on open road with no change in pedal input, I would see the TPS value jump wildly.

Another thing I tried was resetting the throttle. This seemed to fix the problem for short periods. Here are the steps copied from MBWorld.

1. Get in and shut the door
2. Turn key to position 2 (right before the start position) DO NOT START THE CAR
3. Press the gas pedal to the floor... past the kick down point... see why I say DO NOT START THE CAR!??!
4. Hold the pedal there for 15 seconds
5. After 15 seconds while foot still planted, turn the key off but do not remove it
6. Release gas pedal... and get ready to be bored
7. Wait at least 2 minutes while not touching anything in the car, don't open the door even.
8. After the 2 minutes start the car and drive off normally... you should notice a difference in the shift pattern and smoothness.