Downshifting while moving

I had this downshifting problem the other night. I was at a trafffic light light turned green gave it some gass got up around 20-25mph at the truck downshifted and then up shifted and it happen around 35mph also after I gave it some more gas it work fine. Then last night same thing happened downshifted around 20-25mph then upshifted but then the check engine light came on. Has anyone experienced this problem and if so what is the remedy? the truck is a 04 tacoma with 159k on it

 

where do you get a code reader and how does that work

 

Autozone can read the codes for you.

 

thank You but how does the code reader work does it hookup to the engine

 

plugs in to a plug in front of the accelerator pedal. Autozone will know what to do.

 

but yes it does get its info from the ecu. takes a couple mins. nothing major. Its a diagnostic thing

 

Thanks for the info, I'll get by there to day on the way home from work.