Transmission shifts in 3rd for no reason.

My transmission has been shifting into what I think is 3rd gear at random times. Most notably right after hitting the top of a hill and starting back down. When this happened it won't shift at all until I manually put it in 4th then back to drive. Then it acts normal.

 

@gearcruncher may have some ideas on what to check, meanwhile have a look at the thread below.
Do you have a check engine light on and have you checked the truck for codes?

https://www.tacomaworld.com/threads...th-pics-for-second-gen-4-liter-trucks.289913/

 

What speed? What RPM before and after the shift? Stock tires?

 

Its likely the transmission AI downshifting to 3rd to aid in engine braking.

 

Its a computer controlled transmission .
Scan for codes is next . Could be a sticking solenoid , could be sticking in limp home mode .Only a scan will tell
As mentioned above , it would be nice to know the speed you travel when this happens