My 2005 Civic automatic downshifts from 1 or 2 gears by itself, even at high speeds, without my slowing down - even on the freeway. I took it to a transmission shop and they could not find anything, and there are no warning lights. Any suggestions?

 

You may be feeling the torque converter going in and out of lock up. This can sometimes feel like a down shift.

 

Gear engagement is controlled by many things on the engine, particularily engine load and throttle position. A lot of things are used to determine what gear the transmission should be in for a given load and speed.

For instance, traversing a large grade at a speed of 60mph will net at least one downshift, if not two, to keep the engine from over-laboring the hill and to maintain your speed.

More information will be needed to diagnose this problem, if it is a problem at all. Sometimes it is normal operation, but that all depends on when it happens and if you can repeat it.

To eliminate to torque converter, which does feel like another gear in most compacts, will usually disengage when the throttle is fully released. Once the throttle is pressed again, the engine will wait for the throttle to stabilize, and then engage the torque converter clutch.

What gear you were in, what speed, engine temperature, kind of terrain, during cornering, etc.

Here is a helpful article I wrote about automatics:
https://honda-tech.com/forums/transmission-drivetrain-127/automatic-transmissions-torque-converters-explained-2086594/