I had the same problem happen to me, but it got worse in that over a week i had hard shifting. i think it was a combo of a bad solenoid and the EGS (Transmission computer).
Not saying yours will get that way, but if you can get a used replacement EGS for cheap (perhaps a wrecking yard?) it might solve your problem. A slightly newer revision one worked on mine and has been about 4 or 5 months and still no issues, a lot searching on the internet revealed that there are updated
Oh and if you do decide to swap the EGS out make sure you disconnect the battery first.