I had a similar problem within 1 month of buying my truck around 41K miles the tranny started to go in and out of second gear, like it was thinking what it wanted to do...
The problem with these auto's is that they are not electronically controlled and are controlled by hydraulic pressure...Which is the reason a lot immediately switched out the crappy solenoid to the GM style solenoid to increase hydraulic flow....once the stock solenoid gets clogged up with particles from the tranny, you have a tranny time bomb b/c it resricts the hydraulic flow needed to shift the gears..
My tranny was completely rebuilt...new lever, new springs, upgraded and added clutch packs, new solenoid, better bands, better discs basically upgraded everything:rock:
Sounds like you're too late in the game to just add a better solenoid...you need a complete rebuild and it's not cheap if you dont have an extended warranty! Upwards of $2K to do it right!