Highway Downshifting Gears. on 1992 Toyota Paseo

I drive a few city streets and then jump onto the highway when going to work every morning. When I finally hit the highway, it will switch from the highest gear to the next lowest gear, making my engine rev higher than it should. After driving a few miles, it will then resort back to the highest and most efficient gear. I'm wondering why it is normal at first, then jumpsdown a gear, clearly after the engine is already warmed up, and after it has used the highest gear on the highway a few minutes before.

Asked by for the 1992 Toyota Paseo
engine load. if you are on a upgrade, it will downshift to lower gear till the load decreases and go back to the high gear.

The part of the highway on which it automatically downshifts on me is perfectly flat. I have to drive for about 2 miles before it finally shifts back into the top highway gear. When it downshifts on me, I'm usually going about 60 miles an hour, and my RPM's are a bit over 3200. It's sucking all my gas, killing my miles per gallon.
you may have a bad switch for the converter clutch in the valve body. take it to a trans shop to have them check it for you and give you a bid.