Truck sometimes starts off in 2nd gear??

[99GMCZ71]

Every once in a while my 99 NBS Sierra will start off in 2nd gear when the lever is in overdrive, then quickly shift back to first and then shift through the gears fine. Has anybody else every heard of this. The truck has 95000 miles on it and as far as I know the trans fluid and filter have never been changed. I plan on doing that in the near future. Any thoughts?

[Markeaux]

If you search this board for 2nd gear start (or similar) you will probly get a lot of hits ... my 01 did this just a few times, but while there was no specific TSB on it an updated computer flash seems to have fixed it.